The monoclonal antibodies AGF4.48 and AGF4.36 raised against the promyeloid cell line HL60 recognise a fucosylated N-acetyl lactosamine determinant. This oligosaccharide sequence has been shown to be present on a variety of tissues at different developmental stages. Using the immunoperoxidase technique and the AGF4.48 and AGF4.36 antibodies on formalin-fixed, paraffin-embedded tissues, the distribution of the corresponding antigenic determinants during human renal embryogenesis has been studied. Both antibodies bind to the surface of the cells of the ampullae of the ingrowing ureteric bud branches, but not to the remainder of the ureteric bud. Reactivity at this site persists until after fusion of the ureteric bud with the S-shaped tubule, but is then lost. The determinants are also found on different segments of the proximal convoluted tubule in the foetal and adult kidney. The determinants are thus found on the cells responsible for induction of renal tubulogenesis, and separately at specific stages and functionally distinct sites on the developing tubule.