To determine the effects of sex steroid hormones on the growth of an aetiologic agent of chromoblastomycosis, we studied the dematiaceous fungus Phialophora verrucosa. The in vitro growth of this species on culture media containing either progesterone, testosterone or oestradiol at various concentrations was assessed. Both progesterone and testosterone inhibited the growth of P. verrucosa, whereas oestradiol did not. In other experiments, fungal cytosolic fractions were obtained and steroid binding assays were performed. These studies showed that the presence of progesterone receptors possessed two binding sites as determined by Scatchard analysis, one of which has a high affinity to progesterone (Kd = 6.02 x 10(-8) M) with total binding sites of 120 fmol micrograms-1 protein. These findings suggest that the growth of P. verrucosa is regulated by steroid hormones and that the effect of progesterone could be mediated through fungal intracellular progesterone receptors.