Signaling through the CD40 receptor activates diverse molecular pathways in a variety of immune cell types. To study CD40 signaling complexes in B cells, we produced soluble CD40 cytoplasmic domain multimers that translocate across cell membranes and engage intracellular CD40 signaling pathways. As visualized by fluorescence microscopy, rapid transduction of recombinant Antennapedia-isoleucine zipper (Izip)-CD40 cytoplasmic domain fusion protein (Antp-CD40) occurred in both the DND39 B cell line and human tonsillar B cells. Upon cellular entry, Antp-CD40 activated NF-kappaB-dependent transcription, induced proteolytic processing of p100 to the p52/NF-kappaB2 subunit, and increased expression of CD80 and CD54 on the surface of B cells. Antp-CD40 transduction of B cells did not, however, activate detectable levels of p38 mitogen-activated protein kinase or c-Jun N-terminal kinase and did not up-regulate CD95 expression. Analysis of Antp-CD40 complexes recovered from transduced B cells revealed that Antp-CD40 associated with endogenous TRAF3 and Ku proteins. Multimerization of Antp-CD40, or extensive clustering of transmembrane CD40, diminished the disruptive effect of the T254A mutation in the TRAF2/3 binding site of the CD40 cytoplasmic domain. Taken together, these results indicate that Antp-CD40 mimics some of the natural CD40 signaling pathways in B cells by assembling partially functional signaling intermediates that do not require plasma membrane localization. We present a novel approach for delivering pre-activated, soluble receptor cytoplasmic domains into cells and recovering intact signaling complexes for molecular analysis.