In addition to their role as peptide binding proteins, MHC class II proteins can also function as signal transducing molecules. Recent work using B cells expressing genetically engineered truncated MHC class II molecules has suggested that signaling through the cytoplasmic domains of these proteins plays an important role in the generation of signals required for the activation of some T cell hybrids. Treatment of truncated Ia-expressing B cells with cAMP-elevating agents corrects the deficiency in Ag presentation by these cells. We report that the MHC class II-mediated signal appears to act by a mechanism that increases the efficiency of Ag presentation by B cells thereby lowering the amount of specific Ag required for T cell activation. We further show that the induction of the cAMP-induced signal in B cells is inhibited by cycloheximide and cytochalasin A, implicating protein synthesis as well as cytoskeletal rearrangements in Ag presentation to accessory signal- dependent hybrids. In contrast, these agents do not block Ag presentation to a T cell hybrid previously shown not to require the cAMP-induced signal for activation. The signal-dependent T hybrid is additionally dependent on LFA-1-ICAM-1 interaction for activation, whereas the signal-independent hybrid is not. These observations suggest the existence of two types of T cell hybrid with respect to their requirements for activation: those that require only the recognition of MHC class II-peptide complexes without accessory signals, as shown by their ability to respond to purified Ia on planar membranes, and those that, in addition to recognition of MHC II/Ag, require LFA-1-ICAM-1 interaction and the delivery of additional signal(s) induced in the B cell via signal transduction through MHC class II molecules.