Previous studies have shown that head kidney macrophages isolated from glucan injected rainbow trout (Oncorhychus mykiss) and Atlantic salmon (Salmo salar L.) have increased ability to kill Aeromonas salmonicida. The present work was aimed at investigating the in vitro effects of glucan on the respiratory burst and bactericidal potential of Atlantic salmon head kidney macrophages. Salmon macrophages were incubated for 1-7 days with various concentrations of yeast beta-glucan (MacroGard) and tested for respiratory burst activity by the reduction of nitroblue-tetrazolium (NBT) after exposure to phorbol myristate acetate (PMA) or opsonized zymosan. The macrophages showed a marked increase in respiratory burst activity 4 to 7 days after addition of glucan. Macrophages treated with 0.1-1 microgram mL-1 gave a maximum respiratory burst response, whereas 10 micrograms mL-1 had no effect and 50 micrograms mL-1 was inhibitory. The glucan also triggered respiratory burst activity directly, but this occurred only at relative high concentrations with a maximal effect at > or = 200 micrograms mL-1. The validity of using the NBT-assay as a measure of respiratory burst activity was confirmed by using inhibitors of O2- production (superoxide dismutase, trifluoperazine and diphenylene iodonium). Despite the stimulatory effect of glucan on the respiratory burst activity of salmon macrophages, these cells did not show increased bactericidal activity against the avirulent and virulent strain of A. salmonicida. Upregulation of burst activity alone is thus apparently not sufficient to enhance bactericidal activity against this pathogen by Atlantic salmon macrophages.