One or more sulfhydryl (SH) inhibitors usually show greater activity against human and animal solid tumors than against normal tissues in vivo or in radioactive tracer studies in vitro. Selected SH inhibitors inhibit incorporation of thymidine-2-14C into DNA and produce an accumulation of label in thymidine triphosphate in animal and human ascites cancer cells in vitro. The clinically-promising SH inhibitors depress RNA dependent DNA polymerase (‘reverse transcriptase’) activity from Rauscher murine leukemia virus and from lymphoblasts of leukemic patients more than DNA dependent DNA polymerase activity.