Nocardia delipidated cell mitogen (NDCM) was given intragastrically (200 micrograms/animal) to 2-month-old germ-free (GF) and conventionally (CV) reared AVN rats. On day 4, enzymatic activities of enterocyte brush border membrane vesicles (BBMV) isolated from jejunal scraping were measured. The results indicated that activities of sucrase, lactase and glucoamylase in BBMV were stimulated following NDCM treatment. The most pronounced increase of enzymatic activities was found in GF rats. In contrast to CV rats, GF rats do not express MHC class II on epithelial cells during their life. NDCM treatment induced MHC class II expression in enterocytes from GF rats. The levels of IgA and IgG in sera from NDCM-treated CV rats did not change significantly. The level of IgG in sera of GF rats was found to be enhanced after NDCM treatment. Markedly increased incorporation of 3H-thymidine by spleen lymphocytes stimulated with Con A in vitro was observed only in NDCM-treated GF rats. 3H-uridine incorporation into Con A-stimulated lymphocytes of GF rats was decreased after NDCM treatment.