Although the in vitro activity of . aureomycin is of a lower order than that of penicillin for Gram-positive cocci or of polymyxin for Gram-negative bacilli, the new agent is effective against both classes of bacteria. A noteworthy exception to the relative activities of penicillin and aureomycin is the slightly greater effectiveness of the latter for strains of alpha and beta Group D streptococci. Aureomycin is bacteriostatic rather than bactericidal and, since it deteriorates rapidly at temps. over 25[degree]C, its titration end points rise with duration of incubation. Its action in vitro is diminished in the presence of serum.