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The influence of bacteria-free supernatants from cultures of atypical virulent (V234/81, autoagglutinating, A-layer positive) and avirulent (126/68, non-agglutinating, A-layer negative) strains of A. sulmonicidu, obtained after different culture times in yeast-tryptone broth at 20" C, was tested on the PHA response of carp pronephric leucocytes in virro. Supernatants from virulent cultures modulated the response, whereas avirulent supernatants had no effect. The response was enhanced (400%) by supernatant from early virulent cultures (20 h), but severely depressed (~3 % ) by later ones (96 h). The effects were dose-dependent. Inhibitory activity of 96-h supernatant was lost by heating (70" C, 30 min), suggesting that the inhibiting factors are all proteinaceous.Heated 96-h supernatant was as stimulatory as early supernatant. Stimulation of leucocytes also occurred in the absence of PHA with early and heat-treated 96-h supernatants, but at a tenth of the level, suggesting that only stimulated cells (blasts) might respond to the substance(s)present in supernatants. Membrane fragments from virulent and avirulent bacteria, and purified LPS from virulent bacteria, were stimulatory with or without PHA. Endotoxin-free, heat-treated, 96-h culture supernatants were also stimulatory, suggesting that an additional mitogenic factor(s), other than LPS, is present in the supernatants. The modulating in virro effects of extracellular products from A. sulmonicidu might explain the immunosuppression seen during later stages of erythrodermatitis in vivo.