When vincristine sulfate (VCR) was added to serum from mice, rats, dogs, or monkeys, as little as 0.01 vg/ml of VCR could be detected in a KB cell culture system. The bioassay was able to detect 88 to 100 percent of VCR added to the various serum samples. Mice and rats were treated intraperitoneally with 2.9 and 2.0 mg/kg respectively of VCR and bled at specific time intervals following drug administration. Dogs and monkeys were injected intravenously with 1.0 mg/kg of drug and bled at

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... of five minutes to four hours. In mice, dogs, and monkeys, the concentration of VCR was highest (0.3 to 1.0 /ag/ml of serum) at the shortest time interval tested (five minutes) after injection of VCR and decreased rapidly to low but detectable levels (0.02 to 0.05 /.tg/ml of serum) by four or six hours posttreatment. In the rat, the maximum blood level was observed at 20 minutes following intraperitoneal injection of 2.0 mg/kg of the drug and was not detectable at 180 minutes postdrug injection.