Immunohistochemical techniques have been used to investigate the expression of fetal trophoblast antigens and the maternal leucocytic response in molar pregnancy and choriocarcinoma. The antigenic phenotype of morphologically defined trophoblast populations in complete, partial and invasive moles was analogous to that in normal pregnancy. All trophoblast phenotypes described in normal pregnancy were also identified in choriocarcinoma, suggesting that extensive differentiation into heterogeneous subgroups occurs in malignant trophoblast. The maternal leucocytic infiltrate in molar pregnancy consisted of T lymphocytes and class II MHC-positive macrophages. CD2-positive, CD3-negative lymphocytes were identified in molar decidua but not in uterine tissue in choriocarcinoma. Similarly, endometrial granulocytes were present in molar decidua but not in choriocarcinoma; these cells were associated with decidualization rather than with fetal trophoblast.