Microsomal metabolites of aflatoxin B1 (AFB1) causing induction of prophage in lysogenic strain of Pseudomonas aeruginosa SM was studied. Reduction of culture turbidity was determined at various concentrations of toxin. The effect of the toxin was also studied on deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and protein synthesis. AFB1 at the concentration of 50 micrograms/ml reduced initial turbidity to approximately 90% in 4 h. DNA synthesis stopped completely in the first hour but reappeared due to induction of the temperate phage. Soon after induction both RNA and protein synthesis continued but later little or no net synthesis of these macromolecules occurred. Plaque forming units (pfu) were increased approximately 90 times at 2 h as compared to the control. Testing of the effect of AFB1 on the non-lysogenic, sensitive strain demonstrated that although there was no significant decrease in culture turbidity at 50 micrograms/ml concentration of AFB1, DNA synthesis stopped completely within 1 h, while RNA and protein synthesis were increasing throughout the test interval. It has been concluded that the liver microsomal fraction of AFB1 caused induction of prophage in lysogenic cells and inhibited DNA synthesis significantly in non-lysogenic cells.