The roles of interleukin (IL)-1 and IL-2 in bovine B cell function were investigated by determining the ability of each cytokine to support proliferation and differentiation. Recombinant bovine (rb)IL-2, but not rbIL-1 beta, induced the proliferation of B cells. The effect of IL-2 was dependent on the state of B cell activation. Only low and medium density B cells, but not high density, proliferated in response to IL-2. IL-1 enhanced the IL-2-induced proliferation; this effect was maintained if addition of IL-2 was delayed 48 h. IL-1 did not induce the differentiation of B cells into immunoglobulin-secreting cells. IL-2 was an inducer of secretion in all classes of immunoglobulin. As with proliferative responses, only activated B cells were responsive and IL-2 could not induce immunoglobulin secretion by high density B cells. IL-1 acted as a modulator of IL-2-induced immunoglobulin secretion, an effect which was influenced by the timing of the addition of IL-1 and the target B cell population. Simultaneous addition of IL-1 and IL-2 resulted in the inhibition of IL-2-induced IgM secretion, whilst delayed addition of IL-2 inhibited the amount of IgG1 secreted by purified B cells but had no effect on IgM, IgG2 and IgA secretion. IL-1 had no effect on the ability of low- or high-density B cells to secrete immunoglobulins. However, simultaneous or delayed addition of IL-1 maintained or enhanced the production of immunoglobulins by medium-density B cells in response to IL-2.