Treatment of stimulator cells with 0.1% paraformaldehyde for 60 sec or ultraviolet-B (UV-B) irradiation (1000 J/m) eliminates their ability to elicit T cell pro liferation in a primary mixed leukocyte reaction. How ever, a T cell response equal to 20–40% of control value could be elicited by paraformaldehyde fixed or UV-B irradiated cells providing the latter are incubated at 37°C for 18 hr prior to treatment. The incubation also induces a one-log increase in the density of fluorescence when the cells are stained with monoclonal antibodies against class II molecules DR and DP as well as the intercellular adhesion molecule —1 (ICAM-1). We inter pret this as an increase in the membrane expression of these structures following incubation. Chloroquine and cerulenin, known to inhibit protein degradation and antigen processing and presentation do not influence the upregulation in membrane expression of these class II and adhesion molecules, but do prevent incubation from overriding the effect of paraformaldehyde treatment. Colchicine, which reduces the traffic through tubular lysosomes, also has no effect on the upregulation but enhances allopresentation. We propose that incubation of stimulator cells in the presence of chloroquine and cerulenin results in the membrane expression of class II molecules without associated peptides. The inability of stimulator cells expressing such “nude” MHC molecules to elicit T cell proliferation after chemical modification could be due to easier crosslinking of the allodeterminants by paraformaldehyde when the binding site is empty but could also mean that nude MHC molecules are not per se immunogenic and become so only after acquisition of a peptide. It is also possible that chloroquine, NH4C1, and cerulenin block the expression of signals other than the class II and cell adhesion molecules that are essential for induction of T cell proliferation.