Cells from 6 of 14 different Ly-1+ murine B cell lymphomas bound to synthetic liposomes encapsulating fluorescein. The liposomes were made from distearoyl phosphatidyl choline (DSPC), distearoyl phosphatidyl glycerol (DSPG), and cholesterol. In all cases, liposome binding was due to recognition of phosphatidyl choline by the surface IgM on the tumor cells. Liposome binding could be inhibited by DSPC but not by DSPG, and the number of liposomes bound per cell was directly related to the cell surface concentration of IgM. The IgM secreted by a hybridoma derived from one of the lymphomas, CH12, was shown to agglutinate liposomes, and was used in a solid-phase immunoassay to study inhibition of liposome binding by pure phospholipids; DSPC and sphingomyelin both inhibited, whereas DSPG did not. The Ig borne by the six lymphomas that bind phosphatidylcholine also bind to both SRBC and bromelain-treated mouse erythrocytes. The idiotypic of CH12 IgM is similar to that expressed by Ly-1+ normal splenic B cells of the same specificity. The significance of these data in relation to other commonly studied autoantigens, and to the restricted specificity of normal Ly-1+ B cells is discussed.