Hybrids between pseudodiploid Thy-1.1+ lymphomas and Thy 1.2- pseudodiploid Abelson-leukemia-virus-induced-(ALV-induced) lymphomas express Thy-1 glycoprotein on their cell surface. These Thy-1+ hybrids invariably express the Thy 1.1 allelic form of the glycoprotein and may be either Thy 1.2+ or Thy 1.2-. Sublines expressing both Thy 1.1 and Thy 1.2 can be isolated from Thy 1.1+, Thy 1.2- hybrids by cell sorting. In contrast to hybrids with pseudodiploid ALV-induced lymphomas, hybrids between Thy 1.1+ lymphomas and pseudotetraploid Thy 1.2- Abelson-leukemia-virus-induced lymphomas do not express Thy-1 glycoprotein on their cell surface and Thy-1 glycoprotein cannot be detected in detergent extracts of these cells. Thy-1+ revertants were isolated from one of the Thy-1- hybrids by cell sorting. - These results demonstrate a gene dosage effect for the expression of the Thy-1 glycoprotein in somatic cell hybrids. They are consistent with the idea that diffusable gene products regulate Thy-1-glycoprotein expression in these hybrids. They also suggest that there may be additional, apparently cis-active, regulatory mechanisms which determine the ability of the Thy-1 structural genes of the Abelson-leukemia-virus-induced lymphoma parent to be expressed in somatic cell hybrids.