The growth of a strain of Bacillus megaterium was prevented by a minimal 
inhibitory concentration of primaquine of 52 mug/ml or 2 x 10(-4)m. When 
exponentially growing cultures received the drug at 6 x 10(-4)m, the rate of 
growth was drastically reduced and no further growth occurred after 15 min of 
exposure. At this concentration, primaquine was bactericidal, causing a 50% 
reduction in the viable population after one doubling time of 45 min. Supplying 
primaquine to cultures 30 min after adding radioactive-labeled phenylalanine, 
thymidine, uracil, or diaminopimelic acid produced an immediate and complete 
inhibition of protein biosynthesis but no inhibition of deoxyribonucleic acid 
biosynthesis for at least 15 min, and caused the formation of ribonucleic acid 
and cell wall polymer to proceed linearly at rates similar to those established 
prior to the addition of drug. This pattern of inhibition of macromolecular 
biosyntheses suggests that the major in vivo action of primaquine in B. 
megaterium is to block protein synthesis.