Brezhnev appears . . . an heir apparent doesn't

Soviet President Brezhnev, reported unofficially to have been seriously ill, has appeared in public for the first time in a month, Monitor correspondent Ned Temko reports.

But Andrei P. Kirilenko, a Politburo colleague once mentioned by diplomats here as his most likely successor, was absent from the leadership lineup at the celebration of the anniversary of Lenin's birth. A senior Soviet official told the Monitor recently he is ''assuming'' Mr. Kirilenko ''is sick,'' but did not confirm reports to that effect long circulating in Moscow. His last reported appearance at a political function was more than six weeks ago.

Mr. Brezhnev did not address the Lenin celebration, but that was not unusual, since the keynote role rotates among Politburo members. This year that task fell to Yuri V. Andropov, a Politburo figure and head of the KGB.