The inhibitory effect of L-lysine on penicillin biosynthesis by P. chrysogenum was compared in a low-producing strain (Wis, 54-1255) and a high-producing strain (AS-P-78). Lysine inhibited total penicillin synthesis to a similar extent in both strains. In the high-producing strain the onset of penicillin synthesis occurred even at a high lysine concentration, whereas in the low-producing strain lysine had to be depleted before penicillin production commenced.