To assess the potential of CD40 ligand (CD40L) and the related molecules CD27 ligand (CD27), CD30 ligand (CD30L), and membrane TNF-alpha to stimulate B cell responses, expression of these proteins in the baculovirus system was performed. Sf9 cells expressing these membrane molecules were cultured with normal human B cells and a variety of B cell lines to assess the functional outcome. The signal provided by CD40L promotes aggregation of B cells, stimulates vigorous proliferation, and induces germ-line transcription of downstream heavy chain constant region genes in the absence of cytokine costimulation. In contrast, CD27L, CD30L, and TNF-alpha had no effects on B cell proliferation. CD27L and TNF-alpha had no effect on the induction of germ-line transcripts, whereas CD30L consistently inhibited constitutive and CD40L-induced germ-line transcription of the epsilon gene by B cell lines that express CD30. These results demonstrate the various members of the TNF family exert specific effects on human B cell function, with CD40L and CD30L providing powerful, but opposing, effects on l epsilon transcription.