A serum factor which participates in the early steps of the lysis of rabbit erythrocytes (E) by human serum in Mg-EDTA has been partially characterized. Normal human serum (NHS) when adsorbed with rabbit E or stromata at 0°C in the absence of divalent cations, lost 35% of its hemolytic activity against fresh rabbit E. The feedback loop activity (B, D, P) of the adsorbed serum was intact. The lost hemolytic activity was restored with a pH 3.4 eluate from the adsorbed stromata; the activity in the eluate has been termed “0° factor E.”

When 0° factor E was added to serum depleted of 0° factor E by prior absorption with rabbit E, a linear dose response was obtained which was concave to the abscissa, indicating that a single activity was being added. The activity was unaffected by heating to 56°C for 30 min and had an apparent molecular weight of 700,000 on Sepharose 4B gel filtration.

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