Pretreatment with β-1, 3-D-glucan-derivated plastic beads conferred strong protection against Escherichia coli infection in mice. The protective effect showed a dose-response relationship to the amount of beads injected and was dependent on the time point of the injection relative to the infection with E. coli. A similar protection could be obtained in nude mice. Experiments with radioactively labelled bacteria as well as beads indicated a systemic effect of the beads. macrophages extracted from animals treated with glucan plastic beads appeared highly stimulated. This was also true of cells that did not contain beads and presumably therefore not glucan, which seems to indicate a soluble stimulatory factor.