Addition of 10-3 M 6-methyl purine caused immediate inhibition of total RNA synthesis in Chlorella pyrenoidosa, 211-8p, whereas inhibition of total protein synthesis developed progressively with time. 50% inhibition of adenine incorporated into RNA, and of de novo isocitrate lyase synthesis, was caused by 5 × 10-5 M 6-methyl purine, but a ten-fold higher concentration was required to give a similar inhibition of total RNA synthesis. The inhibition of isocitrate lyase synthesis was reversed competitively by adenine. Isocitrate lyase synthesis was inhibited by 91% at a concentration of 10-3 M 6-methyl purine, which gave only 12% inhibition of glucose respiration. It is suggested that 6-methyl purine acts by replacing adenine in RNA metabolism. It is concluded that 6-methyl purine may be used as an inhibitor of functional RNA synthesis in Chlorella and that the synthesis of isocitrate lyase during the cell cycle is dependent upon concurrent transcription.