Photodynamic therapy has possible applications in the treatment of colorectal carcinoma. The photosensitizer haematoporphyrin derivative (HpD) is selectively retained by tumours. Agents which block p-glycoprotein, an export protein expressed to increased levels in a high proportion of colorectal carcinomas, may modulate photodynamic therapy by reducing HpD efflux from cells. Multicellular spheroids derived from the colorectal cell lines HRT18 and HT29 were incubated for 24 h with 1 microgram ml-1 HpD and 0, 1, 2 and 4 microM verapamil. Bioactivity demonstrated a dose-dependent potentiation of HpD-photodynamic therapy growth retardation. Clonogenic survival of cells disaggregated from spheroids treated with HpD-photodynamic therapy was reduced when spheroids were coincubated with verapamil. The mean(s.e.m.) efflux of HpD from spheroids into fresh medium assessed by fluorimetry was greater in spheroids treated with HpD alone (93.2(18.8) arbitrary units ml-1) than in those treated with verapamil (18.1(2.8) arbitrary units ml-1), P = 0.003. Flow cytometry demonstrated increased HpD fluorescence in cells derived from spheroids coincubated with verapamil over a range of HpD incubation concentrations. Verapamil can potentiate the bioactivity of HpD-photodynamic therapy and HpD may be a substrate for p-glycoprotein.