Evaluation of the anti-infectious activity of an immunomodulator performed either in vitro or in vivo in animals as in humans must answer three questions: what are the targets? what models should be used to study the mechanism of action? what methodology should be selected for the assessment of therapeutic benefit? In the case of RU 41470 (Biostim), an immunomodulator with a known structure and of biological origin affects immunocompetent cells and two essential mediators: II1 and CSF. Because of multiple interactions between anti-infectious, anti-inflammatory and anti-allergic responses, as well as the pleiotropism of mediators, there exists no absolute predictive index of activity in vivo and, independently of models of immune deficiency, experimental infections are a particularly useful pharmacological model for study of the anti-infectious activity of any immunomodulator. In this model, RU 41470 tested by oral, intraperitoneal and aerosol administration, proved to be active regardless of the type of infectious agent for extracellular bacteria, intracellular bacteria, viruses or yeasts. Because of special local features of anti-infectious defences (pulmonary, cutaneous), the target organ must be identified when studying mechanism of action. RU 41740 stimulates the metabolic activities of alveolar macrophage and the target organ is the respiratory tract. From a clinical pharmacology standpoint, stimulation of different immune components has been investigated with RU 41470 at different dosages, using double-blind versus placebo designs. Target pathology, regardless of severity, includes a risk of infection and the existence of an immunological deficiency. Chronic bronchitis is a reference pathology since patients are subject to episodes of infection, resulting in acute decompensation and contributing to worsening of the ventilatory obstructive disorder. Clinical efficacy in terms of anti-infectious prophylaxis must be evaluated by a strict methodological approach: randomized double-blind placebo-controlled trials with prolonged follow-up. RU 41470 is effective in prophylaxis of respiratory infections in chronic bronchitis (reduction in the number of respiratory infections, their duration and in antibiotic consumption) and in prophylaxis of respiratory tract infections in children over one year old. Clinicians faced with the perplexity of the mechanism of action of immunomodulators and their number are preoccupied above all by the response which such an anti-infectious immunomodulator can offer in a context of clinical reality.(ABSTRACT TRUNCATED AT 400 WORDS)