In invertebrates the defence system to fight infectious diseases depends mainly on a non-specific or innate immune response, contrary to the vertebrate immune system. The use of natural immunostimulants that enhance the defence mechanism or the immune response of target organisms may be an excellent preventive tool against pathogens. Several strains of baker's yeast Saccharomyces cerevisiae have been found to be good immune enhancers. Previously, it was shown that small quantities of the mnn9 yeast cells and/or glucan particles could protect Artemia nauplii against the pathogenic bacterium Vibrio campbellii in the gnotobiotic Artemia challenge test. Apparently, the higher amount and/or availability of beta-glucans and/or chitin present in mnn9 yeast strain might play an essential role in such protection. The present study reveals that these compounds could only provide protection against the pathogen when they were supplied to Artemia well in advance of the challenge (8-48 h depending on the source). Also the putative immunostimulant did not have a curative action. Moreover, short-time exposure of Artemia to mnn9 strain (priming) did not provide protection against the pathogen longer than two days. Hence, it is hypothesized that the mere stimulation of known biochemical pathways, e.g. prophenoloxidase is not sufficient to explain the mechanisms involved in the observed immunostimulation obtained by beta-glucans and/or mnn9 yeast in Artemia nauplii.