Rats and mice bearing transplantable tumorsshow lower blood levels of 6-mercaptopurine (6-MP) as compared to normal animals. These findings may be explained by the rapid accumulation of 6-MP in the tumor and by the higher capacity of liver homogenates from tumor-bearing in respect to normal animals to metabolize 6-MP. It is also shown that the microsomal fractions of the liver, in the presence of a NADPH generating system, increases the disappearance of 6-MP from theincubation medium.