The stimulation of guinea pig lymphocytes by phytohemagglutinin (PHA), concanavalin A (ConA), methanol-extracted residues of tubercle bacilli (MER), purified protein derivative of tubercle bacilli (PPD), dextran sulphate (DS) and E. coli lipopolysaccharide (LPS), was determined and compared with that of mouse lymphoid cells. The sources of lymphocytes tested were spleen, thymus, lymph nodes and bone marrow. The degree of activation of DNA synthesis by PHA and ConA was higher in guinea pig thymocytes and lymph node cells than in corresponding sources of mouse lymphocytes. The optimum degree of stimulation by PHA and ConA was approximately the same in guinea pig thymocytes, while ConA was by far a better stimulator than PHA for mouse thymocytes. All four B-cell mitogens tested (MER, DS, PPD and LPS) activated DNA synthesis in mouse lymphoid cells while only MER and DS were effective in guinea pig lymphocytes. A guinea pig spleen cell population depleted from B cells was not stimulated, neither by DS nor by MER, while it still responded to PHA and ConA. These results indicate that the proliferative response due to MER and DS occurs in the B-cell compartment. It is suggested that the differences between guinea pigs and mice with respect to their ability to develop a cell-mediated type immunity and to respond to T-independent antigens are related to differences in the relative proportions and degrees of maturation of T- and B-cell subpopulations, as reflected by the selective responsiveness to various mitogens.