An initial separation of reaction stages in immune cytoxicity is described. Primary isolates of normal human amnion cells were reacted first with specific rabbit γ2-globulin antibody, then with human C'1 esterase in the presence of Ca++, and finally with human C'2, C'3, and C'4 in the presence of Mg++. Cytotoxicity, as measured by uptake of trypan blue, occurred only when these reactions were performed in the order given and did not occur when any of the constituents was omitted. Inhibition of immune cytotoxicity was achieved with partially purified preparations of human serum inhibitor of C'1 esterase under certain experimental conditions.

The data are discussed in relation to the role of C'1 esterase in initiating complement action and the partial biochemical definition of an early event in immune cytotoxicity.

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