UV absorption spectrum of polyene antibiotics was dramatically altered in the presence of steroids. The introduction of a steroid to an aqueous solution of polyene antibiotics resulted in a change in the ratio of absorbance of peak 3 (with shorter wavelength) to peak 1 (with longer wavelength), E3/E1. Interactions between 28 steroids and 4 polyene antibiotics were studied and the results revealed that the structures of steroids essential for the optimal interaction with polyene antibiotics were C-17 side chain, substitution C-3 hydroxyl group, intact steroid nucleus and aromatic A-ring. The ratio of E3/E1 was used for the determination of steroids in mycelium of Mucor hiemalis and the result was in good agreement with that obtained from gas chromatograph. The (-) strain had a greater change in the ratio of absorbance, E3/E1, than that of the (+) strain. The ratio of absorbance, E3/E1, decreased in the following order: the (-) strain, the (-) strain with the addition of the filtrate of (+) strains, the mated strains, the (+) with the addition of the filtrate of (-) strain and the (+) strain.