In this study, the mechanisms for suppression of immunoglobulin production by lymphocytes induced by diphenylhydantoin (DPH) were investigated using peripheral blood mononuclear cells from one patient 3 months after discontinuation of DPH, when IgA deficiency was improved. Immunoglobulin production in PBMCs was dose-dependently suppressed by DPH and the suppressive effect of DPH on IgA production tended to be greater than the effect on IgM or IgG production. Moreover, cell separation experiments indicated that DPH suppresses immunoglobulin production by PBMCs through a suppression of B cell, but not T cell, function. The finding that the suppressive effect of DPH on IgA production tended to be greater than the effect on IgM or IgG production might be related to the fact that the alpha-chain gene is located in the 3' end of the gene coding the constant regions of human heavy chains.