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A model system for the investigation of immune cellular injury in primary isolates of a fixed-tissue cell of human origin has been described, using metabolism-inhibition and uptake of trypan blue as independent criteria of cytoxicity. Cytotoxic effects on human amnion cells were produced by the combination of specific rabbit antibody and factors in normal human serum which were indistinguishable from the components of hemolytic complement and calcium and magnesium ions. The data have been discussed in relation to the apparent multiplicity of serologic systems which can effect cellular injury. The nature of the cell and the source of antibody or normal serum constituents have been emphasized as factors which may influence the experimental selection of a given humoral mechanism.

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.