Prostaglandins E2 and F2 alpha (PGE2 and PGF2 alpha) were measured by Gas Liquid Chromatography-Mass Spectrometry (GLC-MS) in extracts of 100 human mammary carcinomas. All tumours contained measurable amounts of both prostaglandins but wide variations between individual tumours were observed. Values for PGE2 ranged from 7 to 762 ng g-1 tissue with a median of 100 ng g-1 tissue. Values for PGF2 alpha ranged from 3 to 475 ng g-1 tissue (median 60 ng g-1 tissue). There was a highly significant positive correlation between amounts of the 2 prostaglandins in individual tumours. Amounts of both PGE2 and PGF2 alpha were not significantly related to the menopausal status of the patients or the presence of oestrogen and progesterone receptors.