Evidence was obtained for the binding of C1q to the membrane of cultured vascular smooth muscle cells derived from human umbilical cord veins. C1q was fixed to the cell membrane at 4 degrees C, whereas it was ingested into the cytoplasm, as a cytoplasmic inclusion, when tested at 37 degrees C. The addition of C1q in advance inhibited the subsequent binding of C1q. Neither fibronectin nor laminin was detected on the cell membrane. Aggregated IgG bound to vascular smooth muscle cells in the case of preincubation with C1q at 4 degrees C, whereas aggregated IgG did not bind to the cells in the absence of C1q. The addition of C1q molecules to the cells in suspension enhanced superoxide generation by vascular smooth muscle cells. There was no effect of C1q on superoxide generation by the cells in monolayer. These results suggest that C1q binds on the membrane of vascular smooth muscle cells via its specific receptor that mediates immune complex binding to the cells and superoxide generation. These properties elucidate the mechanisms by which circulating immune complexes deposit in the vascular wall, and subsequent degradation of tissue components surrounding vascular smooth muscle cells occurs through oxidative burst of the cells.