A high incidence of adenocarcinoma of the colon (greater than 16%) has been observed at necropsy in the South American primate, Saguinus oedipus oedipus (S. oedipus), while the disease has not been found in tamarins of the closely related species, Saguinus fuscicollis spp, housed in the same research colony. Cytogenetic analyses in cultured lymphocytes from 10 S. oedipus and 10 S. fuscicollis illigeri (S. fuscicollis) demonstrated no differences in the average frequencies of spontaneous or mitomycin C (MMC)-induced sister chromatid exchanges (SCEs) between animals of the two species. However, highly significant variability in MMC-induced chromosome lesions was observed between the individual S. oedipus, with one animal exhibiting increased sensitivity for both SCEs and chromosome breakages. At present we do not know the relationship, if any, between increased sensitivity to mutagen-induced cytogenetic lesions in specific S. oedipus tamarins and the increased risk for colon cancer that has been documented in this primate species. However, our cytogenetic findings in this one S. oedipus are similar to data obtained in evaluations of persons with several autosomal recessive conditions in which there is a genetic predisposition for developing malignancies.