Human polymorphonuclear and monomorphonuclear leukocytes (PMNL and MMNL) were exposed in vitro to 8-methoxypsoralen (8-MOP, 0.1-80 micrograms/ml) and/or UV-A radiation (0.03-2 J/cm2) and then analysed for the following functions: chemotaxis, bactericidal activity and proliferation in response to mitogen stimulation. The functions of PMNL became depressed only at a high PUVA dose level (about 20 micrograms/ml of 8-MOP plus 2 J/cm2 of UV-A), whereas with MMNL chemotaxis was inhibited at 1 microgram/ml of 8-MOP plus 2 J/cm2 of UV-A and lymphocyte proliferation was diminished at 0.1 microgram/ml plus 0.1 J/cm2. Since with the MMNL, as compared with the PMNL, a longer time period was present between PUVA exposure and analysis, and since no difference between these cell types in trypan blue exclusion could be seen, the relative sensitivity of the MMNL functions was taken as evidence of DNA damage being a mechanism for the observed PUVA-induced effects.