The aim of the investigation was to throw light on the question whether drugs other than antibiotics and chemotherapeutic agents exert an antimicrobial effect. In order to elucidate this, the antimicrobial effect of selected psychotherapeutic drugs and their stereo-isomeric analogues was studied. The development of psychotherapeutic drugs from aniline dyes has been reviewed against the background of its history considered as a scientific idea. It is demonstrated that psychotherapeutic drugs have an antimicrobial effect. Psychotherapeutic drugs show antimicrobial activity at high concentrations. Stereo-isomeric analogues of known psychotherapeutic drugs also have an antimicrobial effect. The selectivity of the various stereo-isomeric compounds depends on which microorganism and which chemical compound is investigated. Synergism is found between psychotherapeutic drugs (CPZ) and penicillin in vitro, and between a non-neuroleptic stereo-isomeric compound trans-CPT and penicillin in vivo, using infected mice as material. The antibacterial activity of psychotherapeutic drugs is independent of the antihistaminic, antihypersecretory, neuroleptic and antidepressant effect of these drugs. The examples chosen of investigations of the antimicrobial effect of psychotherapeutic drugs in vitro and in vivo lead to the conclusion and to the perspectives in the present study. Namely, the need for a general theory of the interplay between host organism, microorganisms and drugs. This proposition is based on a concern to argue against the view that the prokaryotic effect of eukaryote-directed drugs is without major significance, either for scientific research or for clinical treatment.