Human recombinant tumor necrosis factor alpha (rTNF alpha) and rainbow trout macrophage activating factor (postulated gamma-interferon (gamma-IFN) analogue) synergised to elevate the respiratory burst activity of trout macrophages. This elevated response may parallel phenomena described in mammals where TNF alpha and gamma-IFN commonly synergise. Human rTNF alpha also synergised with mitogenic stimuli to heighten proliferation responses of trout head kidney leucocytes. These data are indicative of a conserved TNF alpha receptor on trout leucocytes (and possibly the TNF alpha molecule itself) and support the notion of an interactive cytokine network regulating immune responses in fish.