TAP-dependent peptide translocation is significantly reduced in primary B lymphocytes incubated with B95.8 supernatant, VFS, or recombinant hIL-10 and vIL-10 in comparison with control cells incubated with cell culture medium RPMI containing supernatant from untransfected Cos7 cells. Primary B lymphocytes were incubated with the different supernatants as indicated for 2 days and permeabilyzed with Streptolysin O, and the ¹²⁵I-labeled peptide RRYQNSTEL was added. Peptides that are translocated into the endoplasmatic reticulum become N-glycosylated at the position NST. Glycosylated peptides were retained on Concanavalin A-coupled sepharose matrix and eluted with α-methyl mannoside, and the eluted radioactivity was determined in a γ-counter. Results are given as ConA-eluted cpm. The addition of apyrase shows that the translocation of peptide is strictly ATP-dependent. The representative results of one of two experiments are shown.