The ability of WR-2721 to protect cultured mammalian cells against radiation-induced killing was nearly the same as that of cysteamine when WR-2721 was activated by mouse liver extract. Without the liver extract, protection by WR-2721 required long incubations with the cells prior to irradiation. The protective activity increased in proportion to the cell concentration. The dose reduction factor at a concentration of 4 mM WR-2721 was 1.11 and 1.41 for 1.5 X 10(5) cells/ml and 15 X 10(5) cells/ml of cultured cells, respectively. A non-protein bound sulfhydryl group was detected in cell suspensions after incubation with WR-2721, but it was not a dephosphorylated product of WR-2721.