After some introductory remarks on the pathogenetic mechanisms of acute undesirable reactions following the administration of antibiotics, the incidence of vascular shock (anaphylactic shock, anaphylactoid shock) is discussed. Investigations were performed to obtain information on the sensitizing capacity of erythromycin and clindamycin in rats and guinea pigs. Although maximation procedures were used, no sensitization could be obtained, in contrast to similar series with penicillin and sulfonamides. In another experimental series, the anaphylactoid properties of erythromycin and clindamycin were studied in vitro. The dye-kick-off test and the mast-cell-degranulation test were used. No pronounced anaphylactoid activity, either of the cellular or of the humoral type, was found in the two antibiotics investigated. The results of the animal experiments and the experiments in vitro are in good accordance with clinical experience: erythromycin and clindamycin rank among the safest antibiotics with regard to the elicitation of acute undesirable actions.