The monoclonal antibodies AGF4 .48 and AGF4 .36 have previously been shown to distinguish human granulocyte lineage cells from other peripheral blood and bone marrow cells. The AGF4 .48 antigen, which is carbohydrate in nature, together with similar antigens described by numerous investigators have been considered specific differentiation antigens of myeloid cells. In immunohistological studies of a wide range of normal tissues, the AGF4 .48 antibody selectively stained cells in several apparently unrelated tissues. These included proximal tubules and descending thin limbs in the kidney, parietal cells in the stomach, a variety of other epithelial cells, astrocytes in the brain, and cells in the anterior pituitary containing adrenocorticotrophic hormone. The AGF4 .36 antibody gave similar results on kidney, stomach and pituitary. These findings emphasise the importance of assessing the binding of monoclonal antibodies, which appear unique in their reactivity with blood cells, to non-haemopoietic tissues before assigning specificity to reagents. The distribution of cells expressing the AGF4 .48 and AGF4 .36 antigen correlates with the occurrence of the 3-fucosyl-N-acetyllactosamine carbohydrate structure in various secreted glycoproteins.