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SUMMARY: A study was made of the binding of aflatoxin B1 and its toxic effects on Bacillus megaterium NRRL B-1368. After a 12 hr incubation period, 5 μg. aflatoxin B1m1. inhibited growth of the test organism 95 %. Viable organisms decreased from 3·8 ⨯ 107/ml. in the controls to 5·0 ⨯ 104/ml. in a growth medium containing 50 μg. aflatoxin B1/ml., after 4 hr. of incubation. Viability was not significantly decreased, however, when the organisms were incubated with the same concentration of toxin either under nitrogen, or at 5°, or in the presence of bacteriostatic concentrations of tetracycline. Binding experiments showed that 4·0 ⨯ 109 organisms/ml. took up approximately 7.0 μg. aflatoxin B1/ml. After five aqueous washes of the organisms, 20-25% of the toxin remained bound to them. The tightly bound toxin was removed by ultrasonic treatment of the organisms and chloroform extraction of the macerate. Removal of cell walls by lysozyme following toxin uptake released 23% of the initially bound toxin, and osmotic rupture of the protoplasts an additional 7%. Toxin was also taken up by intact and by ruptured protoplasts. Removal of nucleic acids from the membranes did not alter their capacity to bind aflatoxin B1.