In view of the changes reported in expression of the different members of the leukocyte-common family of antigens by T cells as they are activated, B cells were examined with monoclonal antibodies against p220-180 (CD45), p220,205 (CD45R), and p180 (UCHL1). Resting tonsil B cells reacted with CD45 and CD45R antibodies, but not with UCHL1. Activation of B cells by anti-IgM and IL-4 and culture of activated B cells with factors which induce proliferation (B-cell growth factor) and differentiation (IL-6) led to only minor changes in the expression of CD45, CD45R, and p180 antigens. Activation with phorbol ester led to minor increases in reactivity with CD45 and CD45R antibodies and led to a weak reactivity with UCHL1. The results suggest that the functional relationship previously described in T cells between p180 and p220,205 is not seen in B cells. p180 is induced, but only at low levels and only with nonphysiologic activation. CD45R is not lost under the same conditions. However, in terminally differentiated B cells, and in some chronic lymphocytic leukemias, p180 expression was seen, often coexpressed with CD45R.