The elution of 35S-labelled C′ from antigen—antibody-35S complement precipitates

C′₁ haemolytic activity and radioactivity were detected in EDTA eluates of immune precipitates formed in the presence of either whole guinea-pig serum or euglobulin fraction labelled with ³⁵S in vivo. Eluted C′₁ and ³⁵S were fixed by sensitized sheep cells which subsequently lysed on exposure to guinea-pig C′ reagents lacking C′₁. The dissociated material contributed only a minute fraction of the protein N taken up by Ag—Ab precipitates from fresh C′.