Flow cytometry was used to show that biologically active N-acetylglucosamine-containing muramylpeptides (GMPs) induced in vitro dose-dependent increase in the expression of tumor-associated antigens (TAAs) characteristic for colon and mammary gland carcinomas, melanoma and lung adenocarcinoma. Forty to two hundred percent enhancement in TAA-expressing cells was observed after 18-48 h incubation with GMPs. In contrast, MHC class I antigen expression was not altered. Using MTT and chromium-release assays, melanoma cells treated in vitro with GMDP were shown to be more susceptible to killing by peripheral blood cells of healthy donors than non-treated cells. Fractionation of blood cells revealed that platelets were responsible for this effect.