delta4-3-Ketosteroids exhibit an intensive negative Cotton effect on the circular dichroism (CD) spectra in the wavelength range for the n-pi* electronic transition (270-350 nm). With hydroxylamine hydrochloride, delta4-3-ketosteroid compounds can be transformed into oxime derivatives. Following oxime formation, positive ellipticity with low intensity can be registered in this wavelength range. The quantitative determination of delta4-3-ketosteroids is based on the considerable difference between the ellipticities before and after oxime formation. The difference ellipticity for the six ketosteroids examined (norethisterone, levonorgestrel, levonorgestrel acetate, methyltestosterone, testosterone phenylpropionate, nortestosterone phenylpropionate) varies linearly with the concentration in the interval 6 x 10(-6)-3 x 10(-3) mol/L. The method can be well applied to determination of delta4-3-ketosteroid contamination of norgestimate [(+)-13-ethyl-17-hydroxy-18,19-dinor-17alpha-pregn4-en-20-yn-3-one oxime acetate]; 0.02-10% impurity can be measured.