Rabbit anticryoprotein and anticomplement antisera recognized a heat-labile antigen in normal human serum. This antigen best fitted the previously described US protein because of its presence in fresh human serum, euglobulin, and purified 11S preparations and its absence in heated serum, R11S, and pseudoglobulin preparations. The 11S hemolytic activity correlated well with the presence of this heat-labile antigen in the 11S region in sucrose density gradient ultracentrifugation and in the gamma globulin region on zone electrophoresis. It could be identified as a single component in the gamma globulin region in immunoelectrophoresis. The intermediate complex EAC'11S was lysed by R11S reagents and agglutinated by these antisera. The antisera also agglutinated a human complement-binding Rh-positive cell system if the 11S protein had been previously bound.

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