The molecular relationship of HLA--D-associated by B lymphocyte antigens was studied in lysostrip, immunofluorescence and MLC inhibition experiments. Two B cell antigens, strongly associated with HLA--Dw2 and Dw3 respectively, redistributed independently of each other in the cell membrane. A more broadly reacting B cell antiserum, which apparently recognized a public antigenic structure present on B cells of different HLA-D phenotypes, coaggregated with the HLA-DW3-associated B cell antigen. Antibodies directed against the HLA-Dw2 and-Dw3 associated cell antigens caused specific stimulating cell inhibition in MLC. Less.inhibition was observed if the target B cell antigen was shared between the responding and stimulating cell donor and thus did not constitute an allogenic difference, indicating that these antigens are MLC stimulating determinants and that they reside on separate molecules in the cell membrane.