N-Propanoylmannosamine is an unnatural precursor of sialic acid, which is taken up by a variety of animal cells and metabolized to N-propanoylneuraminic acid. In several studies it has been demonstrated that application of unnatural precursors of sialic acids such as N-propanoylmannosamine (ManNProp) and homologues interfere with cell differentiation and proliferation of neuronal cells or embryonic stem cells. Since the function of the immune system is known to rely on the presence of sialic acid, we applied ManNProp to human peripheral blood mononuclear cells (PBMC). When culturing those lymphocytes with ManNProp 10 % of the natural sialic acid N-acetylneuraminic acid could be replaced by the newly formed N-propanoylneuraminic acid. This procedure resulted (a) in a marked stimulation in the rate of proliferation of PBMC, (b) a 10-fold increase of IL-2 production coupled with an up-regulation of its receptor CD25 on the cell surface and (c) a concomitant expression and regulation of the transferrin receptor with cell growth. The stimulation of PBMC by ManNProp might therefore introduce a new approach of immunomodulation.