Purified 125I-labelled C1q has been found to react with glutaraldehyde-treated red cells (glut-RBC), with a value for the functional affinity constant, K, of 1-3 X 10(8) M-1, based on measurement of concentrations of bound and free reactants at equilibrium. Values of K obtained for other C1q-binders were as follows: diaminopropane, 2 X 10(2) M-1; monomer IgG, 5 X 10(4) M-1; heat-aggregated IgG, 0-5-2-5 X 10(8) M-1; IgG-anti-IgG complexes, 0-31 X 10(8) M-1. The functional rate constant for association (ka) between 125I-labelled C1q and glut-RBC was 5 X 10(5) M-1 S-1 at 37 degrees in 0-17 M NaC1. The rate of dissociation of the C1q-glut-RBC complex was biphasic with rate constants (kd) of 2 X 10(2) S-1 and 2 X 10(5) S-1. Calculated values of K from the ration ka/kd gave values of 2-5 X 10(7) M-1 and 2-5 X 10(10) M-1. It is suggested that the range of values of K reflects the involvement of 1,2 or more binding sites on the C1q molecule. Reduction of the ionic strength of the medium from 0-17 M to 0-14 M increases the rate of association of C1q and glut-RBC eleven-fold, indicating involvement of ionized groups at the binding site. A method is described for measuring plasma C1q concentrations by saturation assay, using 125I-labelled C1q and glut-RBC. Plasma C1q concentrations fell in the range 170-250 microng/ml.