A new procedure for isolating human C1q from serum or plasma is described. The method, that is highly selective, rapid and involves minimal handling, yields fully active, immunoglobulin-free unaggregated C1q. Several different methods of radiolabeling C1q are compared. These include two methods selective for tyrosine residues, two that label lysine residues, and a method that labels sialic acid residues. The effect of each of the labeling procedures on C1q hemolytic activity was assessed. Also appraised for each method was the ability of the labeled molecules to bind to antibody sensitized cells and to interact with C1r and C1s to form C1. The distribution of each of the radiolabels among the three polypeptide chains of C1q and between the collagenous and globular regions of C1q was determined. Methods were identified that selectively labeled the globular portion of either the A or C polypeptide chain of the C1q molecule without loss of functional activity. Another of the methods labeled all three polypeptide chains relatively uniformly without significant loss of activity.