The interaction of ultraviolet A (UVA) and UVB in producing delayed pigmentation was quantitatively studied on the backs of 19 healthy Japanese subjects. The minimal pigementation dose for UVA (MPDA) and UVB (MPDB) was first determined using narrow-band radiation of 360 nm and 290 nm respectively. The average MPDA and MPDB was 16.4 J/cm2 and 32.9 mJ/cm2 respectively. In experiment 1, fractional doses of 0.2, 0.4, 0.6 and 0.8 MPDA were first irradiated on 4 sites each. Immediately following each exposure, fractional doses of either 0.2, 0.4, 0.6 or 0.8 MPDB were superimposed on the same sites, resulting in each exposure site receiving a different combination of fractions of MPDA and MPDB (A + B). This was repeated with the sequence of exposure reversed (B + A). The results demonstrated that the wavelengths interacted via photoaddition in producing delayed pigmentation, irrespective of the order of exposure. When a 3-h time interval was allowed between exposures in experiment 2, there was again no evidence of deviation from photoaddition with either order of exposure. This study shows that subthreshold doses of UVA and UVB interact additively in the production of delayed pigmentation.