Lymphoid cells of the immune system can be divided into two functional compartments. The thymus derived population, "T" cells, is responsible for cell mediated immunity. The bone marrow derived population, "B" cells, is responsible for antibody production. Although these two populations are functionally different, it has not yet been possible to distinguish them morphologically. Recent experimental work in the mouse has shown that the B cells bear easily detectable immunoglobulin. The T cells can be distinguished by the isoantigen, theta. The B or T cell origin of the lymphocytes of human or animal leukemia has received little attention. In the present study, we have examined the functional and morphologic properties of a guinea pig lymphatic leukemia L2C. L2C cells secrete T2 immunoglobulin and also bear this immunoglobulin on their surface. L2C cells have the recently described lymphocyte receptor for antigen-antibody-complement complexes (found on normal B lymphocytes). Finally, the L2C cell fails to be stimulated in vitro by mitogens capable of stimulating thymus-derived lymphocytes. Thus, the L2C cell appears to be of B lymphocyte origin. The availability of a large number of pure B lymphoid cells will provide a useful tool for the study of the cellular receptors of lymphoid cells and for the preparation of antisera specific for the T cell and B cell populations. The application of the techniques described in this paper to classify other lymphoid neoplasms as to their T or B cell origin may lead to both theoretic and therapeutic advances.