Complement levels of four species of bats were much lower than in guinea pigs when measured by immune hemolysis, bactericidal and immune adherence procedures. Serum pools from three species of Microchiroptera showed somewhat higher levels of complement activity by the immune hemolysis procedure than the megachiropteran species, but not by the immune adherence procedure. Kinetic studies indicated that activities of P. vampyrus and E. fuscus complements were similarly affected by changes in ionic strength of the buffer, but not by changes in pH or in concentrations of divalent ions (Ca++ and Mg++). Eptesicus species of bat complement also gave maximum rates of lysis over a wider range of incubation temperatures than did P. vampyrus and guinea pig complements. Guinea pig complement mediated lysis equally well with rabbit, guinea pig, and bat antibodies. E. fuscus complement showed maximum lytic activity with guinea pig and rabbit antibodies, and P. vampyrus complement with guinea pig antibodies. Minimal lysis occurred when mammalian complements and antibodies were combined with chicken antibodies and complement, respectively. Functionally purified C1 components of guinea pig, chicken, and P. vampyrus and E. fuscus bat complements mediated immune lysis with guinea pig C2–9, but demonstrated little lytic activity in the presence of E. fuscus and chicken C2–9. No significant immunologic correspondence was shown between any of the four species of C1 components by immunodiffusion or turbidimetric procedures. Similar patterns of naturally occurring hemolytic activity present in P. vampyrus and E. fuscus sera for erythrocytes from a wide variety of animals were distinct from patterns obtained with guinea pig and chicken sera. Occurrence of agglutination with some species of cells in addition to or in the absence of hemolysis, however, suggested variations in the specificity of these species of complement for different cell surfaces.