Salmon pronephros macrophages were stimulated in vitro using a novel immunomodulator, aminated beta-1,3-D-polyglucose. Activation of these cells was judged by the measurement of superoxide anion formation, pinocytotic activity, acid phosphatase activity and morphological criteria. Stimulated macrophages showed increased superoxide anion formation compared with unstimulated control cells as measured by reduction of Nitroblue tetrazoleum, and an increased level of acid phosphatase activity. Furthermore, stimulated macrophages showed increased pinocytotic activity and a significant increase in cell diameter and spreading.