ABSTRACT: There is controversy about the size of the major histocompatibility complex antigens of trophoblast cells from placenta. There are some reports that the heavy chains of these molecules are smaller (39–43 kd) than those of the classical class I antigens (45–46 kd), while there are others which show that both the light and the heavy forms of class I antigens occur in the trophoblast cells. In order to investigate this problem, we studied the classical class I antigen (RT1.Aa) and the pregnancy-associated class I antigen (Pa) of the rat from 125I-labeled basal trophoblast cells, isolated from the placenta of WF females pregnant by DA males, using very mild conditions. These antigens were compared with those of the syngeneic (DA × DA) trophoblast cells or paternal (DA) lymphocytes by sodiun dodecyl sulfate-polyacry-lamide gel electrophoresis. Both the Aa and Pa antigens, precipitated from the two trophoblast preparations, showed a heavy chain of 46 kd associated with a 12 kd (β2-microglobulin component, as did the same molecules precipitated from the lymphocytes. Heavy chains in the range of 39–43 kd could not be detected in any of the samples. The results suggest that the smaller molecular weight heavy chains are methodological artifacts and could arise from loss of a glycan(s) during isolation.