The effect of a water-soluble derivative (WSD) of propolis on the classical pathway (CP) and the alternative (AP) complement activity has been investigated. The in vitro experiments show that WSD inhibits both pathways and the effect depends on the source of complement. The suppression of complement-mediated haemolysis proves to be time- and temperature-related. High WSD concentrations cause direct damage of the target erythrocytes. The estimation of C3-residual activity indicates that the preparation diminishes C3 functional activity.