A cDNA encoding a form of hen egg lysozyme (HEL) lacking a leader sequence and predicted to be localized in the cytoplasm, was transfected into MHC class II-positive B lymphoma cells. Cytoplasmically expressed HEL (cytHEL) had a half-life of less than 5 min and did not react with HEL specific mAb suggesting non-native conformation. Cells expressing cytoplasmic HEL, as well as cells previously reported to express a low level of HEL retained in the endoplasmic reticulum (ERHEL), constitutively presented the HEL determinant encoded by residues 46-61 to a sensitive class II-restricted T hybridoma (3A9). Constitutive presentation of HEL determinants was not detectable in cytHEL or ERHEL transfectants using T hybridomas with lower sensitivity to exogenous Ag. Constitutive presentation of HEL46-61 derived from cytoplasmic HEL was demonstrable in multiple transfected clones and was most obvious when a CMV rather than SV40 promoter was used to express the cytHEL gene. The presentation of HEL46-61 by cytHEL transfectants was not due to HEL reuptake by bystander cells because there was no biochemical evidence of cytHEL shedding and cytHEL supernatants added to indicator APC did not result in HEL46-61 presentation. Constitutive presentation of endogenous HEL46-61 by the cytHEL and ERHEL transfectants was inhibited by chloroquine, and recovery of presentation of endogenous HEL was slower in cytHEL compared with ERHEL transfectants. The findings indicate that class II-restricted presentation of Ag retained in the cytoplasm or endoplasmic reticulum does take place but probably requires abundant levels of intracellular Ag and is easily disrupted by lysosomotropic agents. These pathways of presentation may be important when high levels of foreign endoplasmic reticulum-retained or cytoplasmic Ag are present (e.g., viral infection), and during the acquisition of self-tolerance by highly sensitive developing T cells.