A cell regulatory sialoglycopeptide, CeReS-18, purified from intact bovine cerebral cortex cells, has exhibited the capability of reversibly inhibiting cellular DNA synthesis and the proliferation of a wide array of mammalian cells. In the present study, the effect of CeReS-18 on the proliferation of bacterial ( Bacillus cereus and Escherichia coli) and yeast ( Saccharomyces cerevisiae and Schizosaccharomyces pombe) cells was investigated. The results showed that replication and viability of the bacterial cells were not affected by CeReS-18 at any concentration tested, including 15-fold higher than that used for inhibiting mouse 3T3 cell proliferation. In contrast to bacterial cells, CeReS-18 was able to inhibit the replication of yeast cells, in a concentration-dependent, reversible manner, and the addition of calcium to the culture medium could abrogate the inhibitory effect of CeReS-18. A cytotoxic effect of CeReS-18 on both yeast cell species was observed when it was applied at higher concentrations.