1. It has been shown that S-shaped plots of percent lysis versus complement concentration may be produced by probability effects alone in the case of a suspension of identical cells. It is assumed that lysis results when a certain critical number of substrate regions on the surface of the red cell have reacted with antibody and complement.

  2. In the case of identical cells with more than about 50 substrate regions the critical number of reactions required for lysis is about 10 since the slope of the von Krogh plot is 0.20 ± 0.02. The shape of the lysis plot is not very sensitive to heterogeneity if the number of reactions involved is this small.

  3. Equations are presented for treating heterogeneous mixtures of cells and plots of per cent lysis of partially lysed cells.

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