Positively selected human B-cell suspensions with no detectable T cells and containing more than 99.5% B cells both at the initiation and termination of culture were shown to proliferate in response to interleukin 2 (IL-2) in a dose-dependent fashion. The lack of influence of residual T cells on this proliferative response was demonstrated in experiments where T cells were added back in increasing numbers to B-cell suspensions. No detectable enhancing effect on B-cell proliferation was noted when 2.5% T cells were purposely added back to culture, a proportion far in excess of that which might be expected to contaminate B-cell suspensions under the present methodology. In contrast, when 10% T cells were added back to B-cell cultures, an enhanced proliferation of B cells was observed suggesting that the lack of effect of lower numbers of T cells was due to their inefficiency in helping B-cell proliferation in response to IL-2. Therefore, it is concluded that highly purified IL-2 is capable of triggering human peripheral blood B cells to proliferate and that this proliferation is not due to T-cell help.