The effects of 8-methoxypsoralen (8-MOP) monoadducts and crosslinks on growth and viability of mastocytoma cells were investigated. To induce monoadduct formation (4',5'-monoadducts and 3,4-monoadducts), the cells were incubated with 8-MOP (1 microgram ml-1) and exposed to 419 nm radiation, resulting in the formation of more than 96% monoadducts. After washing and resuspension, the cells were exposed to a small dose of long-wavelength UV radiation (UVA, 2 J cm-2) to convert monoadducts into crosslinks. Similar adduct levels were obtained after either 8-MOP plus visible light treatment or 8-MOP plus split-dose protocol. Cells treated with 419 nm light resumed normal growth rates more rapidly than cells which also received the UVA dose. High performance liquid chromatography (HPLC) analysis of DNA obtained from each group of cells showed that the UVA step resulted in an increase in crosslinks from 3.2% after 419 nm radiation to 56.5% after UVA irradiation.