C1 may be reconstituted in macromolecular proenzymatic form by incubation of highly purified C1q, C1r and 125I-C1s together in the presence of calcium. Formation of C1 occurs very rapidly and is essentially complete after 5 min at 0°C. C1 reformed in this manner contains an equimolar ratio of C1 subcomponents and sediments in sucrose density gradients with the 16 S rate characteristic of C1 in serum. Reconstituted C1 is fully activatable as shown by cleavage of the 87,000 dalton polypeptide chain of C1s into disulfide linked subunits of 59,000 and 28,000 daltons, respectively, after incubation with C1 activators. The percentage of C1s conversion and thus the extent of C1 activation may be quantitated. As reformed activatable proenzyme is relatively stable at 0° or frozen, it may be used to quantitatively assess the C1 activating properties of various substances as well as to analyze the C1 activation process.