Rainbow trout blood and head kidney leucocytes were stimulated with the mitogen ConA to release a factor(s) that increased macrophage spreading and adherence. ConA present in control cultures was also shown to have this effect and so removal of ConA was essential to investigate the action of potential lymphokines. Either a 3h pulse with ConA or absorption of supernatants with Sephadex G10 was found to successfully remove activity from control supernatants whilst having no effect upon the lymphokine-containing supernatants.