A type C virus isolate (HL-23V) from leukemic cells is a mixture of baboon endogenous virus (BAV) and simian sarcoma associated virus (SSAV). There is 13% or less homology between BAV 30S RNA and SSAV 30S RNA. RNA (30S) isolated from HL-23 virions appears to consist only of BAV 30S RNA and SSAV 30S RNA. DNA from the spleen or peripheral blood leukemic cells of 25 different leukemia patients, DNA from the spleen, kidney or lung of 15 different patients with other neoplasias and DNA from normal tissues of 14 different individuals were examined for the presence of SSAV or BAV proviral DNA sequences. Cellular DNA excess hybridization to purified 3H-labeled 30S viral RNA revealed that SSAV-RNA or BAV-RNA hybridized to the same extent with leukemic human DNA's and normal human DNA's.