Mycoplasma hyorhinis, isolated by isopycnic centrifugation from supernatants of a persistently infected murine T lymphoblastoid cell line, demonstrated the presence of the Thy-1.1 differentiation alloantigen and H-2Kk histocompatibility antigens. The murine leukemia virus-related gp70 antigen also present on the surface of these lymphoblastoid cells was absent from mycoplasma preparations. Quantitative assessment of Thy-1.1 present in preparations of M. hyorhinis revealed a specific activity greater or equal to that of membrane preparations from lymphoblastoid cells, suggesting a marked accumulation of this T lymphoblastoid cells, suggesting a marked accumulation of this T lymphocyte antigen by membrane-associated mycoplasmas. The accumulation of the Thy-1.1 antigen in association with purified mycoplasmas was also demonstrated in lymphoblastoid cells experimentally infected with a defined culture of M. hyorhinis.