We have used Southern blot analysis to type individuals for the presence or absence of a rare EcoRI RFLP at the c-mos proto-oncogene locus. This polymorphism has previously been reported to be associated with cancer. Ninety-eight patients with non-Hodgkin's lymphoma (NHL) or acute lymphoblastic leukemia (ALL) and 154 cancer-free individuals, including 108 geriatric patients with no family history of cancer, were studied. Because 4 geriatric patients (aged 67-94) were found to have the rate c-mos allele (A2), and the frequency of this A2 allele was no higher among the lymphoma/leukemia patients than among cancer-free individuals, it is unlikely that it constitutes a marker for NHL or ALL.