In oral infections with the yeast Candida albicans, the expression of MHC class II antigens on keratinocytes has been reported to be enhanced. In the present experiments, exposure to C. albicans or its products in vitro was found to increase the expression of class II MHC antigens on thioglycollate-induced mouse macrophages, and on LK cells (an antigen-presenting cell line). The implications of this finding for the understanding of immuno-regulation and susceptibility to C. albicans infection are discussed.