Rabbit, mouse and sheep erythrocytes expressing different concentrations of membrane sialic acid were used to study possible modes of activation of the alternative complement (C) pathway in mouse, human and guinea pig serum. Mouse erythrocytes activated only human serum, whereas rabbit erythrocytes activated the sera of all three species. Based on the observation that rabbit erythrocytes activate the murine alternative C pathway a method for estimation of alternative C pathway activity (AP50 value) in mouse serum was devised analogous to that used for human AP50 determination. The method is not very sensitive to ageing or to batch variation of the indicator cells. The AP50 value of mouse serum measured by this method is of the same order as for human and guinea pig serum. Mouse serum AP50 activity is partly determined by natural anti-rabbit erythrocyte antibodies and is sensitive to heating (15' at 48 degrees C and 4' at 56 degrees C), and to the actions of cobra venom factor, zymosan and cysteine. Strain and sex differences with respect to AP50 activities of mouse sera were observed.