The antigen specificity of a rat monoclonal antibody TER‐119 was investigated. In adult mice, TER‐119 reacted with mature erythrocytes, 20–25% of bone marrow cells and 2–3% of spleen cells but not with thymocytes nor lymph node cells. In fetal haematopoietic tissues, 30–40% of d 10 yolk sac cells, 80–90% of d 14 fetal liver cells and 40–50% of newborn liver cells were reactive with TER‐119. TER‐119⁺ cells in adult bone marrow expressed significant levels of CD45 but not myeloid (Mac‐1, Gr‐1) or B‐cell (B220) markers. Morphological examination and haematopoietic colony‐forming assays for isolated TER‐119⁺ cells revealed that TER‐119 reacts with erythroid cells at differentiation stages from early proerythroblast to mature erythrocyte, but not with cells showing typical erythroid blast‐forming unit (BFU‐E) and erythroid colony‐forming unit (CFU‐E) activities. Erythroleukaemia cell lines do not express the TER‐119 antigen even after stimulation with dimethylsulphoxide. TER‐119 immunoprecipitated protein bands with molecular masses of 110 kDa, 60 kDa, 52 kDa and 32 kDa from erythrocyte membrane, whereas only a 52‐kDa band was detected by TER‐119 in Western blot analysis. Further molecular and cellular analyses indicated that the TER‐119 antigen is a molecule associated with cell‐surface glycophorin A but not with glycophorin A itself.