During classical complement pathway activation, the internal thio-ester of both C3 and C4 becomes exposed which enables C3 and C4 to bind covalently to nearby molecules. Recently, we described that C3 and C4 bind to C1q, the recognition molecule of the classical pathway, upon activation of this pathway. Covalently linked complexes between C1q and activated C4 (C1q-C4 complexes) are specific markers for classical complement pathway activation. In the present study we further investigated the molecular characteristics of complexes between C1q and activated C3 or C4 that occur in vivo. In human serum only complexes of C1q with C3d or C4d fragments were detected but not those with the larger C3b/bi or C4b/bi fragments. We identified that C1q-C4 complexes circulate as part of the intact C1 complex instead of as free C1q. Finally, we investigated whether deposited C3d or C4d affect C1 haemolytic activity. We observed that both C1q-C3 and C1q-C4 complexes are significantly (P<0.05) less active in a C1q-haemolytic assay than non-complexed C1q. Thus, the dominant types of C1q complexes that circulate in vivo are C1q-C3d and C1q-C4d complexes. These complexes are still able to interact with C1r and C1s to form a C1 complex, but seem to have a reduced activity as compared to C1q not carrying C3- or C4-fragments.