Recently, interleukin 1 (IL-1) was shown to protect rats from ARA-C-induced alopecia. The present study was designed to investigate the effect of combination rHu-IL-1 and ARA-C on transplantable chloroleukemia (C51) and at the same time evaluate the protective effect of IL-1 on the ARA-C-induced alopecia in the rat model. In vitro, IL-1 had no direct effect on growth, viability or differentiation of C51 cells, neither did IL-1 protect the C51 cells from ARA-C cytotoxicity. In vivo, the combination treatment of ARA-C + IL-1 aborted the development of transplanted chloroleukemia in 100% of rats, vs 60%, 10% and 0% survival for rats treated with IL-1, ARA-C and buffer respectively. All rates treated with ARA-C alone developed complete body alopecia. In contrast, none of the rats treated with combination IL-1 and ARA-C combination developed alopecia. Thus, IL-1 demonstrated a double beneficial effect, synergism with ARA-C against the leukemic cells on the one hand, and protection from ARA-C-induced alopecia, on the other.