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Kinetic and ultracentrifugal experiments demonstrated that the previously described subcomponents of human C'1, designated C'1q, C'1r, and C'1s, interacted with each other in liquid phase to form a macromolecule which was then capable of converting sensitized erythrocytes (EA) to the state EAC'1. The apparent sedimentation constants of C'1q, C'1r, and C'1s and of the macromolecular product of their interaction were approximately 11S, 7S, 4S, and 18S respectively. Association of C'1 subcomponents was prevented and dissociation of macromolecular C'1 was effected by Na3HEDTA and Na2MgEDTA but not by Na2CaEDTA. The rate of formation of macromolecular C'1 was a function of concentration of subcomponents and temperature of interaction, with an apparent energy of activation of 21,000 calories per mol. Ultracentrifugal studies further indicated the macromolecular nature of C'1 in normal human serum. In the absence of EDTA, C'1 sedimented with the serum macroglobulins and C'1 subcomponents were not detected. Conversely, in the presence of EDTA, macromolecular C'1 was not demonstrable and individual C'1 subcomponents could be measured in lighter fractions. The significance of these observations in relation to previous studies on C'1 subcomponents, the role of Ca++ in C'1 function, and the subunit structure of Enzymes has been discussed.
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