Rosamicin is a new. macrolide antibiotic produced by Micromonospora rosaria. It shares certain chemical and biological characteristics with erythromycin. Activity against gram-positive strains was assayed by broth dilution and compared to that of erythromycin and lincomycin. Rosamicin was bacteriostatic and inhibited most strains of Staphylococcus aureus, Staphylococcus epidermidis, enterococci, viridans streptococci, and group A streptococci in concentrations of 0.02 to 4.0 ,g/ml. Results were similar for erythromycin and for lincomycin (excluding enterococci). Cross-resistance of gram-positive organismsto these three antimicrobial agents was incomplete. Rosamicin was more active than erythromycin against Enterobacteriaceae and Pseudomonas at pH 7.2. Alkalinization of the medium enhanced the activity of both rosamicin and erythromycin; however, rosamicin was still more active than erythromycin against all gram-negative strains at pH 7.6 and 8.0. In view of the high degree of in vitro activity of rosamicin against gram-positive organisms, lack of complete cross-resistance with erythromycin and lincomycin, and the greater activity of rosamicin than erythromycin against gram-negative organisms, further investigation of this macrolide is warranted.