Mast cells in the bilateral testicular biopsies of 30 patients with a 'mixed atrophy' of seminiferous tubules were analysed. Seven biopsies from vasectomized patients served as controls. With regard to their characteristic location within testicular tissue, two groups of mast cells could be distinguished, in both control and infertile patients: 'interstitial' mast cells (located between Leydig and other interstitial cells as well as in the vicinity of blood vessels) and 'peritubular' mast cells (located in the close proximity of the tubular lamina propria or incorporated in the lamina propria itself). Morphometric data indicated a significant increase in the number and volume of mast cells in infertile patients when compared with controls. In the biopsies of infertile patients that were analysed both 'interstitial' and 'peritubular' mast cells showed a significant increase in their number and volume, although it appeared that 'peritubular' mast cells increased at a higher rate than 'interstitial' mast cells. A significant negative correlation was found between the following variables: volume and number of mast cells, testis volume and the status of spermatogenesis evaluated by Johnsen's scoring. It was concluded that the increased presence of mast cells is closely associated with an impairment of spermatogenesis.