The in vitro effects of recombinant interleukin (IL) 5 on proliferation and maturation of mouse Ly‐1 B cells were studied. Most freshly isolated peritoneal Ly‐1 B cells expressed high levels of IL5 receptor (R). Limiting dilution analyses showed that mitogens could reveal IL5 responsiveness in more than half of low density peritoneal Ly‐1 B cells. IL5 was able not only to increase the proportion of these Ly‐1 B cells induced to proliferate, but it also shifted the clone size distribution of proliferating cells towards larger clone sizes. Splenic Ly‐1 B cells also proliferated in response to mitogens plus IL5. Spontaneous and polyclonal activator‐induced plaque‐forming cell responses of Ly‐1 B cells were increased by IL5. Furthermore, IL 5 increased the frequency of peritoneal Ly‐1 B cells induced to secrete certain autoantibodies. IL5 was certainly the agent responsible since its effects on both proliferation and differentiation were inhibited by either anti‐IL5R monoclonal antibodies or by anti‐IL5 monoclonal antibodies. Hence, Ly‐1 B cells, IL5 and the IL5R appear to constitute a system of cellular proliferation, differentiation and some autoantibody production. Strategies specifically targeting the interleukin and receptor elements of this system might afford external control of these cellular responses.