Cnidocytes were isolated from the tentacles and acrorhagi of Anthopleura elegantissima by enzymatic treatment with papain followed by centrifugation in a Percoll-containing medium to produce a concentrated fraction of these cells. The morphology of the isolated cells, as revealed by light and electron microscopy, showed that these cells had intact plasma membranes and was comparable to that of cells in situ. Comparison of the ability of different substances to induce discharge in isolated and in situ cnidocytes showed that the responsiveness of isolated cells was reduced, but not abolished, compared to in situ cnidocytes. Electrophysiological recordings made from cnidocytes isolated from acrorhagi showed that these cells possess voltage-activated ionic currents, further proof that the isolation procedure did not effect the integrity of the plasma membrane. Discharge did not occur with changes in membrane potential.