Studies on the immune effects of antibiotics are in their infancy. Most currently available data fit into the anecdotal category--there are no standard assays for determination of immune inhibition or enhancement. Most studies were performed in vitro; minimal correlation is made between the mode of action and the intracellular penetration of the test antibiotics, and mechanisms of noted effects are largely unexamined and unexplained. Thus, the observations are frequently contradictory and difficult to interpret. Nevertheless, it has become apparent that certain antibiotics do have potent immunomodulating actions. The clinical impact of such immunomodulation is unknown, since few studies have been performed in patients receiving antibiotics and virtually no studies have examined the effect of antibiotics on the immune system of those at highest risk for sustaining an adverse effect, the immunocompromised patient. Well-planned studies on antimicrobial action, adapting the rapidly expanding knowledge of the immune system currently being generated and utilizing the advances in technology for scrutiny of immune affectors, are badly needed. With a better understanding of the effects of such agents on host cells as well as target cells, it is conceivable that in the future antibiotic therapy will consist of regimens that provide not only optimal anti-infective therapy but also optimal immunomodulating support.