The sulphydryl compound WR 2721 has been combined with a range of cytotoxic drugs in the mouse and the effects upon tumours and normal tissues determined. In the acute lethality (LD50/30) assay, mean protection factors produced by WR 2721 (200 or 400 mg kg-1) were generally less than 1.3 for cyclophosphamide (CTX), CCNU and chlorambucil (CHL) but a protection factor of 1.7 was obtained for cisplatinum (cis-P) in combination with 400 mg kg-1 of WR 2721. No protection against the depression of peripheral white cell count seen at 3 days after CTX, CCNU or cis-P was obtained with either 200 or 400 mg kg-1 of WR 2721. Significant protection of the RIF-1 sarcoma by WR 2721 against CTX and cis-P induced growth delay was seen. In the KHT sarcoma, WR 2721 produced small reductions in the growth delay caused by CCNU, melphalan and CHL but these were not statistically significant. These data show less differential normal tissue protection by WR 2721 than do a number of reports in the literature.