The effect of N-acetyl-D-glucosamine (NAG) on in vitro stimulated human peripheral blood lymphocytes was investigated. NAG was added to lymphocyte cultures stimulated by the mitogen PHA, the antigen PPD or to one-way-stimulated mixed lymphocyte cultures from unrelated donors. The results indicate that NAG (1-2 micrograms/ml) in appropriate experimental conditions has an enhancing effect on DNA and protein synthesis induced by PHA, PPD and allogeneic cells. Addition of NAG (1-2 micrograms/ml) to unstimulated lymphocyte cultures had no effect on DNA or protein synthesis.