Thymocyte x PCC4azaR (a murine embryonal carcinoma cell) hybrid cells resembled PCC4azaR. They continued to express two embryonic antigens (SSEA-1 and 3C4-10) found on PCC4azaR, but did not express the thymocyte antigens Thy-1 and Ly-2. However, H-2Kk of thymocyte origin continued to be expressed, while H-2Db of PCC4azaR origin was not reexpressed. This is consistent with H-2 expression being subject to cis control. In syngeneic hosts these hybrids formed tumors containing stem cells and differentiated derivatives. In contrast, an L-cell x PCC4azaR hybrid resembled the L-cell parent, did not express SSEA-1 and 3C4-10, but did express both H-2Kk and H-2Db. In syngeneic hosts this hybrid formed undifferentiated sarcomas.