A pilot study of the endogenous steroid concentrations in human breast tumours was performed. The technique of high-resolution molecular-ion monitoring during combined g.l.c.-mass spectrometry was used to determine oestrone, oestradiol-17beta and oestriol in concentrations above 1ng/g wet wt. of tissue, and dehydroepiandrosterone, testosterone, androsterone (3alpha-hydroxy-5alpha-androstan-17-one) and 3beta-hydroxy-5alpha-androstan-17-one in concentrations exceeding 5ng/g, in extracts of five primary breast tumours.