Platelet-absorbed sera were obtained from placental clots after delivery by multiparous women. These sera contained antibodies that react with PHA-activated lymphocytes after the latter are separated from peripheral blood and expanded with interleukin 2. These alloantibodies did not react with resting T lymphocytes, but reacted with B lymphocytes, PHA-activated lymphocytes, or both types of cells obtained from some but not all of the T lymphocyte donors. Reactions against B lymphocytes were associated with anti-Ia-like antibodies on the basis of blockage by turkey antibodies against human Ia. Reactions against PHA-activated lymphocytes that were blocked by turkey anti-beta 2m were classified as 'HT'. Several antibodies were found to give reactions to HT determinants in separate panels of lymphocytes from Tel Aviv and Boston. The reproducibility of the cytotoxicity reactions was 89%. Altogether, 23 of 1100 sera were found to contain these reactions when screened by a panel of cells obtained from 30 individuals of known HLA phenotypes. Correlation coefficients were determined for all reactions, determining three clusters of significant reactivities: sera 965 and 1032 defined HT-2; sera SF48 and 1642 defined HT-3; and sera 1136, 1605, 1014, and 1227 defined HT-4. HT-2 was found to be inherited with HLA in 11 siblings from four families. Some of these antibodies react with antigens (non-HLA) containing beta 2m that were expressed on activated lymphocytes, but not on resting T lymphocytes, and did not react with thymocytes from the same donors of the peripheral lymphocytes. Our findings suggest that the HT alloantigens expressed on lectin-activated lymphocytes are class I differentiation antigens of a system analogous to the murine Qa system.