The treatment of bacterial prostatitis has always been difficult since only few substances have proven effects in clinical practice. The reason for this was investigated by animal studies in two steps. First a model of experimental bacterial prostatitis was developed. Then pharmacological studies with determination of penetration of various antimicrobial substances into serum, interstitial fluid, urine and prostatic secretion were performed. As a result we could show, that the place of the inflammatory process is the interstitium with only little effects in the prostatic acini. Thus only substances with good concentration in the interstitium can be expected to be effective in the eradication of bacterial invasion of the prostate gland. These are the basic macrolide erythromycin for gram-positive germs, the amphoteric tetracyclines or the newest quinolones for gram-negative bacteria, mycoplasma or chlamydiae.