This is a retrospective study of the records of 17 patients who suffered from infectious endocarditis with positive blood cultures, and concerns the possibilities of predicting immediate infectious signs in patients:--by testing each of the antibiotics used for the minimal inhibitory concentration;--by determining the peak of the serum bactericidal activity of patients. All the patients were under observation, as outpatients, for more than a year and proved completely cured of their infectious endocarditis. A satisfactory minimal inhibitory concentration does not necessarily prevent an infectious evolution in the immediate future. The determination of the peak serum bactericidal activity does not reflect faithfully and instantaneously the anti-infectious action of the treatment; this is shown by the excellent and rapid evolution 13 times out 20 although serum bactericidal activity was considered insufficient (less than 1/8). This level of 1/8 is not a guarantee of therapeutic efficiency. Finally the antiinfectious treatment has been, in the majority of cases, determined by clinical signs and in very few cases by serum bactericidal activity results.