SummaryThe correlation between the results of the sensitivity tests performedin vitroand the true activity of the antibioticsin vivocan be established only if numerous factors arc considered. These factors, such as absorption, binding to plasma proteins and excretion, are reviewed. The kinetic of the bactericidal activity of the treated patients should be measured in the blood.These measurements are indicated in patients with absorptive disturbances. Performed in some 40 individuals, they demonstrated significant bactericidal activity for I.M. penicillin, streptomycin, methicillin, kanamycin, irregularly with peroral erythromycin; chloramphenicol and tetracylines were bacteriostatic. Gross discrepancies were found with amphicillin, staphylomycin and fucidin.