A trial was performed with a long-acting dapsone (DDS) injection, consisting of an aqueous suspension of dapsone crystals, in doses of 900 mg and 1200 mg. Forty-one Ethiopian leprosy patients, 13 women and 28 men, participated in the study. There appeared to be a large discrepancy in the serum concentration curves of dapsone between men and women. Following injection of 900 mg dapsone in men, a peak of 2.28 +/- 1.06 micrograms/ml (mean +/- S.D.) was observed in the first week. After two weeks the serum concentrations had fallen to 0.42 +/- 0.29 micrograms/ml, and after four weeks they fell to 0.11 +/- 0.09 micrograms/ml. Following injection in women, the curves were smooth with a peak in the first week of only 1.04 +/- 0.40 micrograms/ml, while the serum concentrations after four weeks were still 0.42 +/- 0.23 micrograms/ml. The differences between the mean curves of men and women were statistically significant (p less than 0.001). The 1200 mg dapsone injections were only given to men. The explanation of the sex difference in intramuscular absorption can probably be found in the differences in the thickness of gluteal fat in men and women. In these Ethiopian leprosy patients, the non-protein-bound fraction of dapsone comprised 17 +/- 4%. In saliva, 19.5 +/- 7.0% of the dapsone level in serum was found. Methemoglobin levels were raised but did not reach levels of clinical importance. No other significant side effects were observed.