METHOTREXATE, (R)- (D-methotrexate) as a contaminant of commercial methotrexate, a chemotherapy agent and immune system suppressant. Methotrexate from various commercial sources has been found to contain 0.5 to 48% (w/w) of the enantiomer D-methotrexate. D-Methotrexate was found to be a good inhibitor of dihydrofolate reductase from both murine and human tumor cells, but was a poor inhibitor of L1210 and CCRF-CEM cell growth. In animal experiments with dogs and mice, D-methotrexate was rapidly absorbed from the intestine and excreted by the kidneys.