Antisera to sheep erythrocytes (E) were raised in cattle, rabbits, mice, hamsters, guinea-pigs, ferrets, badgers, hedgehogs and fowls. Cross activation of total haemolytic complement (THCA) examined all combinations of sensitized sheep E and normal sera (including human); kinetic assays examined the lysis of E sensitized with rabbit antibodies. From the same species, all combinations of normal serum and xenogeneic E were used to measure total alternative pathway activity (TAPA); TAPA was also activated by rabbit and sheep E in titrations and in agarose gels, and examined kinetically against rabbit E. Ox, rabbit and fowl sera were low in THCA, guinea-pig complement was universally active, while human complement showed marked selectivity; ferret, badger and hedgehog sera were activated to high titres but probably via the alternative pathway. In studies of TAPA an inverse relationship existed between serum complement activities and the activating abilities of E from the same species. The most efficient activators of alternative pathway were E from rabbits and laboratory rodents, while the sera with broadest response were badger, ferret and fowl. Kinetic studies of TAPA showed that initiation of lysis and subsequent completion of lysis could occur with different efficiencies, suggesting these events reflected separate events in complement activation.