The differentiation of T and B lymphocyte subpopulations, both before and after stimulation with T-dependent or T-independent antigens, is being studied. Particular attention has been paid to the state of reactivity of neonatal mouse lymphocytes. Two major B cell sublines have described based upon their reactivity to antigen, surface immunoglobulin expression (surface IgM greater surface Igd, and vice versa), expression of the Lyb5 differentiation antigen, and presence in the CBA/N mutant subline mouse. Some T-independent antigens have been found to cause priming for enhanced antibody responses following injection into newborn mice; this priming results from antigen interacting only with one immature B lymphocyte subpopulation.