Background.Small, resting B cells (rB cells) express major histocompatibility complex (MHC) class II molecules but not the putative costimulatory molecules, B7-1 (CD80) and B7-2 (CD86); they are classified as nonprofessional antigen-presenting cells. rB cells have been shown to be capable of anergizing T cells in vitro and inducing the prolonged survival of skin grafts mismatched for a single minor histocompatibility (miH) antigen, H-Y. The aim of this study was to investigate ability of rB cells to induce unresponsiveness to multiple miH and MHC antigens.Methods.Mice were pretreated with 1 × 107donor rB cells 14 days before transplantation of cardiac grafts mismatched for either a single or multiple miH and/or MHC antigens in vivo.Results.rB cells induced indefinite prolongation of cardiac grafts mismatched for H-Y antigen (C57BL/10 male to female). Moreover, 50% of grafts mismatched for multiple miH antigens (C3H to CBA) were accepted indefinitely in recipients treated with donor rB cells. In marked contrast, when grafts were mismatched for either a single MHC class I antigen, Kb(CBK to CBA), or multiple MHC and miH antigens(C57BL/10 to C3H), pretreatment with rB cells did not prolong graft survival. To investigate why rB cells were ineffective tolerogens for grafts mismatched for MHC antigens, we examined the fate of the cells in vivo. We demonstrate that, after intravenous injection of rB cells, expression of B7-2 was induced within 24 hr.Conclusions.These data suggest that rB cells may be less effective at inducing specific unresponsiveness to MHC antigens because of their rapid activation in vivo.