A subset of endogenously synthesized Ags can be processed for class II-restricted presentation, probably through multiple mechanisms. Processing of exogenous Ags for class II-restricted presentation appears to occur in unique endosomal processing compartments with lysosomal characteristics including the presence of the lysosomal membrane protein LAMP-1. Therefore, we attempted to enhance the efficiency of class II-restricted presentation of an endogenous Ag, the HIV-1 envelope (env) protein, by specifically targeting the Ag to class II processing compartments through the pathway followed by LAMP-1. Because the env protein associates tightly with CD4 shortly after synthesis, we first targeted the env protein using a chimeric CD4 protein consisting of the extracellular domain of CD4 and the transmembrane and cytoplasmic domains of LAMP-1. When co-expressed with this chimeric protein, the env protein was efficiently localized to lysosome-like compartments. Enhanced stimulation of env-specific CD4+ T cell clones by APC expressing the env protein and the CD4-LAMP-1 chimera was readily demonstrated in both cytotoxicity assays and proliferation assays. We also targeted the env protein directly as a chimeric protein consisting of the extracellular domain of the env protein and the transmembrane and cytoplasmic domains of LAMP-1. The proliferative response of env-specific CD4+ T cell clones to the env-LAMP-1 chimera was greatly enhanced compared with wild-type env protein, especially when limiting numbers of stimulator cells were used. The enhanced stimulatory capacity of APC expressing LAMP-1-targeted Ags has important implications for vaccine design.