We have studied the effects of cocultivation on the frequency of mitomycin C (MMC)-induced chromosomal aberrations. This was carried out by cocultivating Fanconi anemia (FA) cells from the genetic complementation groups A and B with both normal mouse lymphoma L5178Y cells and the derived "FA-like" mutant cells, MCN-151 and MCE-50, assigned to complementation groups I and II, respectively. The results show a partial complementation of the defect (i.e. a reduction in the frequency of chromosomal aberration) in FA group A cells cocultured with normal or group II mouse cells, and a partial correction of mouse group I cells cocultived with normal or FA group B human cells. No reciprocal effects were observed between FA group A cells and mouse group I mutant cells; the frequencies of MMC-induced chromosomal aberrations in these cells remained unchanged by cocultivation. Moreover, no complementation was observed for both FA group B cells and mouse group II cells, after cocultivation with normal cells of either mouse or human origin. This implies that a diffusible factor released by normal human and mouse cells, and by FA group B and mouse group II mutant cells, can correct at least in part the chromosomal defect of FA group A and mouse group I mutant cells. With normal human or mouse cells, the frequency of chromosomal breakage after cocultivation remains the same as that observed in non-cocultived cells. This suggests that no detectable clastogenic factor is released by human FA or "FA-like" mouse cells.