Halothane was investigated for its mutagenic effects in Drosophila melanogaster. The induction of sex-linked recessive lethals was used as an indicator of genetic damage. Adult male flies were exposed to halothane either for 14 days at 1,000 or 1,600 ppm (v/v) or for one or two days in 2,100 or 20,000 ppm. In several experiments slight increases of the mutation frequency were observed. For the pooled data of the 14 day-exposure experiments, the increase amounted to a doubling of the spontaneous rate, just reaching the level of 5 per cent significance. The authors consider this a borderline result, indicating, with a fair degree of probability, that halothane has weak mutagenic activity, under the conditions studied.