The immunological and cytochemical phenotypes of five primary feline lymphomas and six feline lymphoma lines are reported. Thymic lymphomas induced by the Rickard strain of FeLV (FeLV-R) are of prothymocyte or (immature) cortical thymocyte origin, as these express terminal deoxynucleotidyl transferase, the guinea pig erythrocyte rosette receptor, Ia antigens, partial cortisone sensitivity, and nonspecific esterase. Lymphomas associated with other strains of FeLV form rosettes with guinea pig erythrocytes, frequently have Ia antigens and cytoplasmic nonspecific esterase, and probably originate from helper T-cells, monocyte/macrophages, or null cells. These data belie previous conclusions that FeLV leukemogenesis is restricted to mature T-cells; rather, the considerable heterogeneity in the surface and cytochemical phenotype of feline lymphomas probably reflects transformation of multipotent lymphoid or monocytoid precursors in the bone marrow by FeLV.