The mouse monoclonal antibody (mAb) BMA031 (IgG2b) has recently been described to be directed against a monomorphic part of the human T cell receptor (TcR) alpha/beta. In vitro analysis of its stimulatory potential for mononuclear cells revealed two patterns of responsiveness. Out of 35 tested individuals only 2 generated a proliferative response to low antibody concentrations (15 ng/ml; "high responders"), the others ("low responders") responded only to high antibody concentrations (1.5 micrograms/ml); the anti-CD3 mAb UCHT1 (IgG2b) stimulated only the two high responders. This response pattern to BMA031 was determined by the accessory cell compartment in the culture. Stimulation by BMA031 in low responders demonstrated some unusual features: (a) high antibody concentrations were required, (b) addition of autologous serum had no inhibitory effect and (c) vigorous depletion of macrophages reduced but did not abolish the proliferative response. These characteristics were shared by two other mAb, BMA032 and BW239/347, presumably directed against the TcR alpha/beta but not by several other antibodies to the TcR/CD3 complex. Thus, the results demonstrate unusual stimulatory properties of three anti-TcR alpha/beta mAb, inducing a proliferative response without antibody cross-linking. This suggests that the stimulatory effect of anti-TcR/CD3 complex mAb is not only determined by their isotype, but also strongly depends on their epitope specificity.