Both calcitriol and UVB radiation exert potent antipsoriatic effects. We hypothesize that the therapeutical effect of UVB radiation may be attributed at least in part to UVB-triggered cutaneous synthesis of calcitriol. The optimum wavelength for initiation of the vitamin D(3) pathway was found to be in the range of 300+/-5 nm in vitro and in vivo. The narrowband Philips TL-01 lamp which is commonly used as UVB source for phototherapy of psoriasis has maximum spectral irradiance at around 311 nm which is presumed to be, however, of lesser importance in photochemical activation of the vitamin D(3) pathway. The aim of this study was to compare the vitamin D(3) and calcitriol-inducing potential of UVB from the TL-01 lamp with that of monochromatic UVB at 300+/-2.5 nm and 310+/-2.5 nm in organotypic cultures of keratinocytes supplemented with 25 microM 7-DHC. We found that maximum calcitriol-generating capacity of the TL-01 lamp at 500 mJ/cm(2) and 16 h after irradiation still amounts up to 44% of that found after monochromatic irradiation at 300+/-2.5 nm and 30 mJ/cm(2). Thus, the antipsoriatic effect of UVB emitted from the TL-01 lamp may, at least in part, based on the antiproliferative and prodifferentiative action of newly synthesized calcitriol on epidermal keratinocytes.