A quantitative rosette assay was used together with various pharmacologic probes to study the interaction between IgG-sensitized sheep erythrocytes (EIgG) and the rabbit alveolar macrophage Fc receptor. Of the pharmacologic agents examined, cytochalasins (A > E > D > B) produced the most marked inhibition of rosette formation. None of the agents consistently augmented resetting. The cytochalasin effect was not explicable by a direct effect on EIgG or by reduced macrophage adherence or viability. Cytochalasin modulating activity occurred within a few minutes and did not increase by preincubation of the monolayers with cytochalasin for up to 60 min. Cytochalasins did not disrupt preformed rosettes. The inhibitory effect of Cytochalasins B and D was reversible with washing, whereas the effect of cytochalasins A and E was not. Vinblastine and colchicine (10-5 M only) also produced modest inhibition of rosette formation. Among the agents producing little or no inhibition of rosette formation were dibutyryl cAMP and cAMP agonists, 8-bromo cGMP and cGMP agonists, sulfhydryl-containing and sulfhydryl-reactive compounds, heparin, ethanol, and polyanions and polycations. The data suggest that cytochalasin-sensitive membrane structures, most likely microfilaments, are important in the interaction between IgG-coated particles and the macrophage Fc receptor.