The interaction between soluble immune complexes and the first component of complement (C1) was studied. Complexes were prepared from purified bovine thyroglobulin (BTg) or tetanus toxoid (TT) and immunospecific IgG antibodies. Purified human precursor C1 was incubated with dilutions of the preparations, and the inhibition of C1 haemolytic activity was determined as a measure of C1-binding. The activation of C1 was assessed by measuring the amount of C4 consumed by generated C1. The molar antibody/antigen (Ab/Ag) ratio of BTg--anti-BTg mixtures strongly influenced their C1-binding and C1-activating capacities: mixtures with high Ab/Ag ratios were by far the most efficient. On the other hand, the Ab/Ag ratio had only a limited influence on the activity of TT--anti-TT complexes. The effect of complex size was investigated by ultracentrifugation of antibody-antigen mixtures on calibrated sucrose density gradients followed by C1-binding and -activation experiments with the fractions obtained. For both types of immune complex, the C1-binding and -activating capacities increased markedly with increasing complex size. Thus, both the size and the Ab/Ag ratio of soluble immune complexes influence their capacity to activate the classical complement pathway. The effect of the Ab/Ag ratio, however, may also be dependent on the antigen molecule(s) present in the complexes.