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Summary Sheep erythrocytes were coupled with trinitophenyl sulphonate, sensitized with anti-TNP (or-DNP) IgM monoclonal antibodies, and exposed to components of the classical pathway of complement activation. When human fibronectin (FN) was added after C1q, but before addition of C1r and C1s (subunits of the first complement component), inhibition of haemolytic activity was observed which was strictly dependent upon the dose of FN. When FN was added after addition of C1 (reconstituted from C1q, C1r and C1s), the haemolytic activity of complement was not affected by the presence of FN. These data suggest that FN binds on C1q by interfering with C1r and C1s fixation. In addition, FN was unable to displace the activated subcomponents (C1r and C1s) from their binding site on C1q. When using other systems (sheep erythrocytes sensitized with anti-Forssman IgM monoclonal antibodies), the quantity of FN required to inhibit complement haemolytic activity was greater than in the TNP system. In normal plasma, there is a 50-fold excess of FN compared to free C1q.