Mouse skin beta-irradiated 3 months earlier shows increased content of a 30-50,000 MW mitotic inhibitor and decreased susceptibility to an exogenous inhibitor.

A 30,000-50,000 MW extract of mouse skin beta-irradiated 3 months previously with 3,000 rads, exerted a stronger in vivo inhibitory effect on mitoses in the basal layer of normal skin than a similar extract from unirradiated skin. Furthermore, epidermis beta-irradiated 3 months previously showed a reduced mitotic rate and a decreased susceptibility to inhibition by the 30-50,000 MW extract. The pathogenesis of the long term effect of irradiation might include interference with the functioning of a local mitotic inhibitor.