AbstractProtamine induces a gradual change in plasma membrane permeability in rabbit neutrophils, which is evident from the increase of indol fluorescence, and the leakage of quin2 from quin2‐loaded neutrophils. The influx of extracellular Ca2+into the neutrophil provides an explanation for exocytosis which occurs in the presence of Ca2+and protamine. The dependence of exocytosis on Ca2+concentration follows the same pattern as is observed in neutrophils permeabilized by other means. In the absence of Ca2+, and in the presence of protamine, La3+has an activating effect on exocytosis. At higher concentrations La3+inhibits exocytosis that occurs in the presence of Ca2+and protamine, as do some other metal ions. The resemblance between the membrane effects of a number of toxins, as reported in literature, and protamine‐induced membrane damage suggests that they occur via the same mechan