Aliquots of human blood mononuclear cells were cultured in vitro in the absence or presence of varying concentrations of histamine (10(-3)-10(-8) M) for 24 hr, mitomycin treated, washed, and cocultured with autologous indicator cells that were then stimulated by PHA. The degree of suppression of thymidine uptake was measured in the presence of histamine-activated cells. The results demonstrated that histamine, in a dose-dependent fashion (10(-3) and 10(-4) M), significantly suppressed the PHA proliferative response. A minimum of 2 hr was required to activate cells by histamine to express suppressor function. While initial studies were carried out with a 1:1 suppressor:indicator cell ratio, it was found that maximum suppression resulted with lower ratios (1:2 or 1:5) in some experiments, and was dependent somewhat on the concentration of histamine used. That the suppressor cells expressed histamine receptors was shown by the finding that histamine-Sepharose columns (but not control columns) depleted the histamine-reactive cells. In addition, Con A-reactive suppressor cells also adhered to histamine columns. Based on experiments with H1 and H2 antagonists and agonists, the histamine-responsive cell was apparently activated through its histamine type-2 receptor.