Summary. This study examined the in vitro effect of a commercial immunoglobulin preparation on human monocytes and the Fc-receptor dependent binding of antibody coated platelets. Monocytes were exposed to Sandoglobulin in vitro and subsequently examined for membrane surface bound IgG. Dramatic increments of surface IgG were found which were maximal with 18 h exposure and somewhat higher at 4°C than 37°C. Ultracentrifugations of Sandoglobulin immediately prior to monocyte exposure reduced the monocyte membrane IgG by 75%. The 18 h exposure at 37°C produced dramatic impairment of monocyte Fc-receptor binding of IgG coated platelets (P < 0.001) while exposure for 18 h at 4°C produced a modest impairment of Fc-receptor function. These studies indicate that Sandoglobulin contains IgG aggregates which are able to firmly bind to the monocyte surface in a time and temperature dependent fashion. The dramatic impairment of Fc-receptor function at 37°C and not at 4°C suggests that Fc-receptor modulation, as well as competitive inhibition/steric hindrance, contribute to impairment of monocyte Fc-receptor function.