Serologic and immunochemical techniques were used to characterize plasma membrane antigens of the murine lymphoma 6C3HED. Syngeneic C3H/HeJ anti-6C3HED antisera were prepared and were shown by cytotoxicity, absorption, and immunoprecipitation tests to be specifically reactive with the lymphoma cells. Alloantisera to H-2.23 were found to be unreactive with intact lymphoma cells by cytotoxicity and absorption assays. However, H-2.23 antigens were detectable in tumor cell lysates by inhibition of cytotoxicity and immunoprecipitation. SDS-PAGE electropherograms of immunoprecipitates of radiolabeled tumor cell extracts obtained with anti-6C3HED and anti-H2.23 antisera were essentially identical; proteins with Mr of approximately 70,000, 45,000, and 12,000 were apparent in both. An immuno-adsorbent of insolubilized anti-6C3HED removed H-2.23 antigens detectable in lymphoma extracts, but had no effect on H-2.23 antigens in normal splenocyte extracts. These data indicate that H-2.23 and TAA are physically associated on 6C3HED cells.