Seven thymus-derived lymphomas in culture were inhibited, as measured by RNA or DNA synthesis, by low concentrations of phytohemagglutinin (PHA), a T-cell mitogen; 8 myelomas were resistant to PHA. Both classes of tumors grown in fetal calf serum were inhibited by concanavalin A (Con A), another T-cell mitogen. Higher concentrations of Con A were required to affect cell lines in horse serum, and lymphomas were generally more sensitive than myelomas. Pokeweed mitogen and the B-cell mitogen Salmonella typhosa lipopolysaccharide, at concentrations which stimulate lymphocytes to divide, did not affect the tumors. All cell lines were sensitive to ricin, and some lgG2a myelomas were selectively sensitive to extracts of Ulex europeus. Inhibition of cellular DNA or RNA synthesis by PHA was blocked by N-acetyl-galactosamine, inhibition by Con A was blocked by methyl-mannoside, and inhibition by ricin was blocked by galactose and partially by N-acetyl-galactosamine. These lectin effects provide alternative markers to cell-surface differentiation antigens for the characterization of T and B lymphocytes and tumors.