We have previously shown (J. Exp. Med. 1984. 160: 1170) that murine B cells activated with lipopolysaccharide (LPS) and rabbit anti-mouse Ig antibodies together (LPS-RaMIg blasts) express high-affinity interleukin 2 receptors (IL 2R) and respond to IL 2. This is not the case for B cells activated with either LPS alone (LPS blasts) or anti-Ig alone. In the present study IL 2R function and expression were further investigated by using this model. First, it was found that LPS-RaMIg blasts internalize IL 2 in a time- and temperature-dependent manner very similar to that occurring in CTLL (T lineage) cells. LPS blasts, however, did not internalize IL 2. LPS blasts were found to express 12 times less binding sites for anti-IL 2R monoclonal antibody (PC 61 monovalent Fab) as compared to LPS-RaMIg blasts and at least 30 times less IL 2 binding sites of high as well as of lower affinity. Second, with regard to the requirements for receptor expression, it was observed that either anti-Ig or phorbol diester (phorbol-12-myristate 13-acetate) can induce IL 2R and IL 2 responsiveness (proliferation assay) in LPS blasts but not in fresh B cells. Taken together these results provide further evidence for the similarity of IL 2R function in activated B and T cells, confirm that surface-Ig cross-linkage and phorbol-12-myristate 13-acetate have similar effects on B cells and suggest that LPS on the one hand, and phorbol-12-myristate 13-acetate or anti-Ig on the other provide complementary signals necessary for IL 2R expression.