The correction of chromosomal hypersensitivity to mitomycin C (MMC) in Fanconi anemia (FA) human lymphoblasts is observed by growth in a medium conditioned by normal human cells. Under the same conditions, the cytotoxic effect of MMC on FA cells is restored to an almost normal level. The addition of interleukin-6 (IL-6) to an unconditioned culture medium increased the resistance of FA cells to MMC cytotoxicity. This correcting effect is partially abolished by addition of an anti-IL-6 antibody to the conditioned medium. Both lymphoblasts and fibroblasts derived from FA patients demonstrate a reduction in IL-6 production. Moreover, this lymphokine is not induced by tumor necrosis factors alpha and beta (TNF alpha and TNF beta) in FA cells, as is the case in normal cells. It is suggested that the observed deficiency in IL-6 production may account for one of the major characteristics of FA disease, i.e., the defect in differentiation of the hematopoietic system.