A technique is described for the culture of slices of benign prostatic hyperplasia (BPH) for periods of a week in organ culture. Under these conditions tissue repair took place, resulting in a covering layer of transitional epithelium which formed around the explant and spread out laterally as a monolayer. Autoradiography and studies with [(3)H]thymidine uptake suggested that the repair activity, which reached a peak at Day 3 in culture, was the centre of biochemical activity, overshadowing that of the rest of the explant. Necrosis of the explant base tended to develop abruptly during the first day of culture but thereafter remained stable. The epithelium was well preserved morphologically, but explant acid phosphatase activity fell progressively.No morphological response to testosterone (10(-5) mol/l) or stilboestrol diphosphate (10(-5) mol/l) was seen.Attention is drawn to a possible source of misinterpretation of results offered by the uptake of [(3)H]thymidine into DNA in organ culture.