Oncogenes of retroviruses are directly related to cancers in animals; however, finding relevant associations with human cancer has been difficult. Studies reported here were designed to assess the extent of protooncogene transcription in human leukemias as compared to normal cells. Low-stringency hybridization of viral oncogene DNAs to cell messenger RNAs was done to determine whether evolutionarily divergent oncogene sequences were increased in leukemic cells. Several oncogenes hybridized at low levels to normal messenger RNAs. Some oncogene transcripts in fresh leukemic cells of 28 patients were present at higher levels than in normal cells. The oncogenes abl, erb, myc, and ras were expressed in all normals tested and a majority of the leukemic cells. However, fes, fps, and src probes hybridized rarely to transcripts in normal cells although more frequently to messenger RNA from leukemic cells. No oncogenes were expressed in specific patterns by cell types. It is concluded that transformation of human hematopoietic cells involves more than one oncogene and may include sequences unrelated to retroviruses.