Cells of patients with Fanconi's anemia (FA) are characterized by their high mitomycin C sensitivity. This specific response was used to study the question of heterogeneity in cell hybrids. After fusion of somatic cells of different FA patients and a normal control, the resulting hybrids were cytogenetically analyzed with respect to their mitomycin C susceptibility. Complementation--indicating heterogeneity--should lead to normal amounts of mitomycin C-induced chromosomal damage. No complementation was found in hybrids between cells of a classical FA patient and one without skeletal malformations. However, clear evidence for heterogeneity was observed in hybrids between cells of the latter patient with early onset and another with late onset of the disease. This confirms the assumption of Schroeder and coworkers based on the high intrafamilial correlation for age at onset.