The in vitro activity of the ester Ro 41-3399 and its free active acid Ro 40-6890 was tested against 189 strains of anaerobic bacteria in comparison to other oral cephalosporins and to antimicrobial agents established in the treatment of anaerobic infections. Prevotella, Porphyromonas, Peptostreptococcus, Fusobacterium and Clostridium spp. were susceptible to Ro 40-6890, with few exceptions. Due to its lack of activity against the major pathogens of the Bacteriodes fragilis group, Ro 40-6890 does not promise to be of major use in the treatment of infections caused by anaerobes.