A rapid 96-well plate assay using sulphorhodamine B (SRB) protein stain for cell number has been adopted to screen herbs used in traditional treatments of vitiligo for substances capable of stimulating melanocyte proliferation. Its applicability to melan-a cells, a mouse pigmented cell line, has been validated. SRB assay produced good linearity up to 11 x 10(4) cells/well and interference by melanin present in the cells accounted for less than 10% of the total optical density readings. The intra-assay variation was small but interassay variation was marked. For better assay precision, it is recommended that the results to be compared should be performed on the same day and controls should be plated in the same experiment, ideally in the same plate. Optimum conditions for exponential melan-a cell growth were established: viz. initial plating density (3-8 x 10(3) cells/well), incubation period (4 days) and foetal bovine serum concentration (5%). Under these conditions cells were responsive to the mitogen tetradecanoyl phorbol acetate (TPA). Out of 28 herbal extracts screened in this assay, significant stimulation (P < 0.05) of melanocyte proliferation was observed, in the absence of TPA, using aqueous extracts of Astragalus membranaceous root, Citrus reticulata peel, Dictamnus dasycarpus root bark. Ophiopogon japonicus root, Poria cocos sclerotium and Tribulus terrestris fruit.