An oncornavirus was discovered in a subllne of the continuous human cell line, Detroit-6. Electron microscope studies of the cells revealed particles of types A and B or theta. The former were found in various parts of the cytoplasm, but the latter were predominantly in the intercellular spaces. Biochemical and biophysical tests were used to study both extracellular and intracellular virus. The virus was immunologically distinct from the mouse mammary tumor virus and shared a common antigen with the virus isolated from HEp-2 cells and with the rhesus monkey mammary carcinoma virus.