Preincubation of skin with colchicine had the following effects:

1. Darkening induced by MSH was increased in comparison to control skins, and on removal of MSH, lightening was inhibited. Inhibition was a function of both concentration (1 x 10^–5 to 9 x 10^–5 M) and exposure time (2 to 30 minutes). Once established, inhibition was maintained throughout the remainder of the experiment. 2. The same effects were noted (a) when darkening was effected by agents other than MSH (ATP) 0.9 x 10^–3 M; caffeine, 5.2 x 10^–3 M; ethyl acetate, 0.8 x 10^–2 M), and (b) when lightening was effected by addition of chemical agents (melatonin, 4.3 x 10^–10 M; hydrocortisone, 1 x 10^–3 M; norepinephrine, 1 x 10^–3 M), instead of by washing. 3. Colchicine alone produced a gradual, irreversible, dosage-dependent darkening over several hours. This darkening was inhibited by melatonin, 4.3 x 10^–10 M.

The melanocyte model is used to construct a general theory of colchicine action on living cells, an action resulting in decreased protoplasmic viscosity. In this formulation colchicine lowers the potential limit of protoplasmic gelation, and does it rapidly, reversibly, in low concentration, in a dosage-dependent manner, and without killing the cell. The theory allows interpretation of "synergism" and "antagonism" to colchicine by other substances. It suggests a tentative approach to the understanding of colchicine action in acute gouty arthritis, where interference with ameboid activities of polymorphonuclear leukocytes is one possible aspect of the anti-inflammatory effect of colchicine. Finally, the colchicine-treated melanocyte is viewed as a good, live physical model that can be used to elucidate some fundamental biological properties.