Immunocytological investigations were performed on semi-thin sections of human fetal pituitaries 6-26 wk old. Corticotrophs were revealed by anti-ACTH, anti-ACTH and anti-.beta. MSH [melanocyte stimulating hormone], but not by anti-.alpha. MSH immunosera from the 8th wk. Somatotrophs were revealed with anti-human STH [somatotropic hormone] from 9th wk. Differentiating cells containing only .alpha. subunits of glycoprotein hormones were present 8-12 wk. At 13th wk .beta. subunits of TSH [thyroid stimulating hormone] were revealed immunocytologically in thyrotrophs. .beta.-Subunits of LH [luteinizing hormone] or FSH [follicle stimulating hormone] were detected in some gonadotrophic cells only from 15th wk.