Study of 26 cases of osteo-articular infections with an identified organism collected in a rheumatology department, has confirmed in the first place the increasing importance of gram negative organisms, compared with the staphylococcus which has always mainly predominated in this type of infection. In each case, the authors have made as complete a bacteriological study as possible, including antibiotic sensitivities, the measurement of the minimum inhibitory concentration of the antibiotics on the isolated strains, study of the bactericidal power in vitro of various combinations, and study of the inhibitory power of the serum on the organism. This study of the inhibitory power of the serum approximates most closely to testing the antibiotic activities in vivo, since one cannot make this assessment, in most cases, within the infective focus itself. Rifampicin, with the imperative condition that it is not used alone, is a major antibiotic in the osteo-articular infections, by reason of its very great activity against the staphylococcus, its excelent penetration into bone, and its ease of use. Other recognized anti-staphylococcal antibiotics behave in a more disappointing fashion. When the effectiveness of treatment is checked regularly by the inhibitory power of the serum, together with the other classical criteria of surveillance, cure of the infection as been achieved in every case.