Washed, progesterone-induced mycelium of Fusarium solani transformed 17α-hydroxy-4-pregnene-3,20-dione (17α-hydroxyprogesterone) into its 1-dehydro analogue, 1,4-androstadiene-3, 17-dione (androstadienedione), and traces of 17β-hydroxy-1, 4-androstadien-3-one (1-dehydrotestosterone) and 17α-oxa-androsta-1,4-diene-3, 17-dione (1-dehydrotestololactone); steroids saturated at C-1 and C-2 could not be detected. However, washed non-induced mycelium ( i.e., previously grown in the absence of steroid inducers) converted the same substrate into its 1-dehydro analogue, androstadienedione, as well as steroids saturated at C-1 and C-2, 17β-hydroxy-4-androsten-3-one (testosterone) and 4-androstene-3,17-dione (androstenedione). The mycelial 1-dehydrogenation and side-chain degradation enzymes of F. solani involved in these oxidative reactions appear to be inducible. The simultaneous addition of a number of antifugal or antibacterial antibiotics and the steroid substrate to washed mycelium previously grown in absence of steroid inducers inhibited the induction, and hence the transformation. The induction of the enzymes is a function of inducer structure and concentration, as well as duration of exposure. It appears that an unsubstituted 17α-hydrogen, an intact 17β-methyl ketone side-chain, and a Δ 4-3-keto function on the steroid nucleus are necessary for maximal induction. Enzyme induction in spores could not be observed. The enzymes of 1-dehydrogenation and side-chain degradation from the spores of F. solani appear to be constitutive. Steroid transformations with spores produced in absence of steroid inducers were unaffected by any of the antibiotics used even after treating the spore with phenethyl alcohol to ensure their permeability to the antibiotics. The spores transformed the same substrate into its 1-dehydro analogue and little of androstadienedione. The steroid metabolising activity of the spores dropped dramatically upon germination and disappeared completely from mycelia collected in the logarithmic or stationary phase of growth.