The 40 kD monocyte Fc receptor for IgG is capable of binding murine IgG1 and of supporting an IgG1 anti-T3 T lymphocyte proliferative response among approximately 80% of Caucasian individuals (responders), whereas the 40 kD Fc receptor on monocytes of the remaining individuals (nonresponders) is incapable of interacting with murine IgG1. By using a monoclonal antibody (mab IV3) that reacts with the 40 kD receptor, we found that the monocyte 40 kD receptors from responder and nonresponder individuals cannot be distinguished by either electrophoretic mobility on SDS-polyacrylamide gels, or by the number of receptors per cell as determined by indirect immunofluorescence. However, isoelectric focussing of the purified radioiodinated 40 kD receptor revealed that the monocyte receptor from all of four nonresponder individuals evaluated has a single distinctive pattern of multiple, regularly spaced bands, whereas the pattern of the 40 kD monocyte receptor from 11 responder individuals is of two sorts. One (seen in four of 11 responders) consists of multiple, regularly spaced bands that are asynchronous with the nonresponder pattern, and the other (seen in seven of 11 responders) consists of multiple bands that correspond in mobility to all of the bands of both of the other two patterns. The incidence of these three patterns suggests that the 40 kD Fc receptor is encoded by a single structural gene with two alleles, both of which are expressed.