The difficulty of directly isolating the casual agent (by obtaining specimens in situ or if necessary by hemoculture), during infectious osteo-arthritis and mainly during those deeply located such as spondylodiscitis and sacro-iliac infections, has led authors to investigate the field of serologic examinations. In staphylococcosis, which is quite frequent, they report results of serum anti-alphastaphylolysin dosage (AASTL) in 500 controls, 100 patients in whom a staphylococcus was recovered and 14 patients with other germs. In the first group, 10 subjects out of 500 ad a level greater than 2 IU whereas in the second group an elevation of the level of AASTL was observed in 66% of cases of staphylococcal osteo-arthritis (35 out of 53) and in 54% of cases of staphylococcosis without esteoarticular localization (25 out of 47), a total of 60% of staphylococcosis with various localizations (60 cases out of 100). Despite the elevation AASTL serum in 4 of the 14 cases of the third group, this study confirms the great diagnositc value of this dosage mainly when the levels are higher than 4 IU or are still rising. The recent study of the serum antigammastaphylolysins represent an additional factor of interest because of its reliability and its frequent positive result during staphylococcosis (in 19 cases of osteo-arthritis out of 27, and in 10 cases of staphylococcosis without osteo-articular localization out of 16: a total of 29 cases in a series of 43 staphylococcosis). The dosage of both AASTL and AGSTL, however, seems more valuable since out of 40 cases of staphylococcosis, we have observed a simultaneous elevation of AASTL and AGSTL in 22 cases (55%), a sole elevation of AASTL in 8 cases and AGSTL in 5 cases (12.5%). Therefore, in this series, at least one of the enzymes was elevated in 15 cases out of 40 (87.5%) which represents an increase of 12.5% when compared with the study of the sole AASTL and an increase of 20% when compared to the study of AGSTL.