We have examined leukocyte DNAs obtained from 21 patients with various types of leukemia for the Sacl polymorphism of H-ras-1. The patients included ten with B-lineage leukemia, two with T-cell leukemia, and nine with acute myelogenous leukemia. Using a genomic probe for H-ras-1 in Southern hybridizations, we found that 78% of the DNA from leukemias of myeloid origin were heterozygous. In contrast, only three of ten B-cell-derived leukemias showed heterozygosity. This difference in the number of heterozygotes vs. homozygotes in the two patient samples studied is statistically significant (P = 0.04 by Fisher's exact test). These results suggest the possibility that the H-ras-1 allelic pattern that is associated with acute myelogenous leukemia is distinct from that found in B-cell-derived malignancies.