Lipoprotein from the outer membrane of Escherichia coli is a potent mitogen towards lymphocytes of different species. A lipopeptide fragment corresponding to the TV-terminal part of the lipoprotein was prepared by chemical synthesis. The mitogenicity of the compound was demonstrated by measuring the incorporation of both [3H] thymidine into DNA and [3H] uridine into RNA at various times after the addition of lipopeptide to mouse spleen cell cultures. The lipopeptide also activated B-lymphocyte development into immunoglobulin secreting cells, as shown by a hemolytic plaque assay. The mitogenicity of the synthetic compound was comparable to the activity of the B-lymphocyte mitogens lipoprotein and lipopolysaccharide. Thus, it was shown that a fragment of a bacterial surface component prepared by chemical synthesis, exhibits a marked biological activity.