We examined the effects of CD86 ligation on surface densities of CD80 on activated B cells. For this, splenic B cells where CD86 levels had been enhanced upon anti-immunoglobulin M (IgM) treatment were employed. Incubation of such CD86-expressing B cells with anti-CD86 monoclonal antibody (MoAb) resulted in a marked upregulation in cell surface levels of CD80. Such an enhancement in surface levels was also accompanied by a marked increase in CD80 mRNA, suggesting that the effects of CD86-triggering are exerted at the level of CD80 message. Furthermore, coculture of CD86-upregulated B cells with CD28-expressing fibroblasts also led to the enhancement of CD80 levels, which was not obtained upon incubation with cells expressing cytotoxic T-lymphocyte-associated molecule (CTLA)-4-Ig. Finally, we also demonstrate that CD86-induced CD80 is fully competent to function as a costimulatory molecule, as revealed by its ability to enhance cytokine secretion by allogeneic T cells. Thus, these results reveal the existence of another pathway that regulates the expression of CD80, at least on activated B cells, which could have important implications in directing the course of an immune response.