Hairless mice were exposed to UVB irradiation from a Philips T1 12 light source. Mice that were exposed to UV-light and not protected with 5% p-aminobenzoic acid (PABA) showed a significantly higher number of peripheral blood granulocytes (P less than 0.001) and a significantly higher mean weight of both the spleen and the liver (P less than 0.001) than the non-irradiated controls. Light microscopy of the histology of the enlarged liver and spleen showed a proliferation of granulocytes and the reticuloendothelial cells. Treatment with topical PABA during the whole period of UV-exposure prevented the peripheral blood granulocytosis. These protected mice also had a significantly (P less than 0.001) lower mean weight for the liver and spleen than UV-exposed and non-protected mice.