RECENTLY I spent a few days in Baltimore's inner city. Driving away on a late afternoon, I saw a public-service billboard in an empty lot with a message that surprised me. In huge red letters the billboard proclaimed: VIRGIN: Teach your kid it's not a dirty word.Three cheers for the message that abstinence is OK. Even Magic Johnson has seen the light and jumped on the abstinence bandwagon. But because teenage pregnancy is about as common as rap music here, is this public-service billboard just a feeble David with no sling shot facing a sex-charged Goliath? The answer is no. David in this case is Hal Donofrio of Richardson, Myers & Donofrio, a Baltimore advertising agency. With private funds, money from the state of Maryland, and plenty of support from Gov. William Schaefer, Mr. Donofrio and his associates designed a multimedia campaign and classroom program, including counseling, to stop young kids from having sex. Known as the "Campaign For Our Children," it is the first of its kind. In a phone interview Donofrio said, "We did some research and learned that 6 percent of kids between 10 and 12 had had intercourse in the last four weeks, but that 80 percent of kids 14 and younger were sexually abstinent. We wanted to get them to extend their abstinence." The project was triggered by a group of pro-life and pro-choice advocates in the state legislature. They put aside their differences and said, "What can we do to try to stop the soaring rate of teenage pregnancies in the state?" When all costs were added, teenage mothers were costing Maryland a staggering $454 million a year. Baltimore was No. 1 in the nation for births to girls 14 and under. In a society where sexual images bombard kids daily, what Donofrio is doing is to effectively neutralize those images. With 27 years experience in media and a desire to "stop the terrible waste in lives," he targeted boys and girls from 9 to 14 with a campaign that costs a modest $1 million a year. The ad campaign - TV and radio commercials, billboards, newspapers, buttons, a 24-hour hotline - works in conjunction with three lesson plans in the schools and material for parents. He also insisted on a five-year program with criteria established to measure the effectiveness of the campaign. One poster shows a close-up of a baby's face. The poster says, "It's amazing how many guys disappear when one of these shows up." At the bottom of the poster is the line, "Be a man. Be responsible." A T-shirt says, "You can go farther if you don't go all the way." At the Lemmel Middle School, students in one class kept a list of activities such as going to a movie, visiting friends, playing sports, and sleeping late. Then they drew a line through each one if taking care of a baby would limit such activity. Three years after this unique campaign was launched, what are the results? Although Donofrio realizes precise measurement is impossible, he and state officials believe the program is working. "At five middle schools," said Donofrio, "research indicated that 94 percent of the kids were aware of the program and could repeat messages and slogans verbatim. Also, 75 percent of the kids said the campaign had helped them talk with their parents about sex and family life. The national average for that kind of discussion within that age group is 20 percent." Most important, the Maryland Department of Health reported a 5 percent reduction in the state for teen births in l989 and another 5 percent in l990. In l989, there were 16 percent fewer abortions for teenage girls. "We are thrilled with the results," said Donofrio. "We think we have just saved the state around $45 million based on these figures, and that the potential is there to save the state millions more. When a child has a baby, she starts a cycle of poverty that can last generations." The "abstinence" word is spreading. Donofrio says 46 other states and three foreign countries have inquired about the campaign.