N-Palmitoyl-(S)-seryl-(S)-seryl-(S)-asparaginyl-(S)-alanine (Palmitoyl-tetrapeptide) is an analogue of the N-terminal part of the lipoprotein from the outer membrane of Escherichia coli . It was prepared by chemical synthesis and tested for biological activity in in vitro lymphocyte culture systems. In spleen cells of the inbred mouse strains C3H HeJ , C3H/He/Bom/nunu, and Balb c , the compound exhibited stimulatory activity towards B-lymphocytes comparable to the effect of native lipoprotein, as measured by the incorporation of 3H-thymidine and 3H-uridine, and by a hemolytic plaque assay. The B-lymphocyte tumor cell line BCL1 was also activated by the compound. The results demonstrate, that the N-terminal tetrapeptide moiety of lipoprotein, linked to a lipophilic molecule, constitutes by itself a novel B-lymphocyte mitogen.