Cultured human fibroblasts accumulated methotrexate polyglutamates to levels far in excess of the dihydrofolate reductase binding capacity. After four days in methotrexate-free medium the intracellular drug level dropped by 70% but nearly 80% of the remaining methotrexate was in the form of polyglutamates. Reduced folates prevented the accumulation of polyglutamates and the effects of methotrexate on deoxyuridine incorporation into DNA and cell growth if present along with methotrexate from the beginning of the incubation. However, the reduced folates were less effective if added to cells after a long exposure to methotrexate alone. Thymidine, glycine, and adenosine (GAT) prevent methotrexate toxicity only if maintained in the incubation medium. However, preincubation with methotrexate and GAT permits continued synthesis and accumulation of polyglutamates so that when the GAT and methotrexate were removed, toxicity from the retained methotrexate polyglutamates could be expressed. (2,4-diamino-5-(3'4'-dichlorophenyl)-6 methyl pyrimidine (DDMP), an antifol that does not form polyglutamate derivatives, inhibited deoxyuridine incorporation into DNA as long as the DDMP remained in the culture medium. Compared to what was seen with longer exposures to methotrexate, removal of DDMP resulted in a greater reversal of the inhibition of deoxyuridine incorporation.