Local bacterial infections such as abscesses or purulent exudates most often contain numerous, easily culturable bacteria despite an intense inflammatory reaction characterized by the ingress of polymorphonuclear leucocytes. In order to understand the mechanisms leading to such a persistent infection, we used pleural empyema as a model and measured the levels and catabolism of complement as well as of immunoglobulins in 28 infectious pleural effusions associated with either a positive or with a negative bacterial culture. Classic and alternative pathway haemolytic activities, factor B and C4 haemolytic activities as well as native C3 were markedly decreased or undetectable in most culture-positive effusions when compared to culture-negative effusions (P less than 0.005); breakdown products of factor B and C3 were markedly increased in culture-positive fluids. Eleven out of 14 culture-positive fluids exhibited IgG breakdown as opposed to none of the culture-negative fluids. In seven out of 14 culture-positive fluids, incubation with 125I-IgG led to their in vitro breakdown. This proteolytic activity could be abolished by preincubation of the culture-positive fluids with normal sera. Thus, increased catabolism of complement and breakdown of immunoglobulins, both leading to local consumption of immune reactants, could be one of the causes for bacterial persistence in pleural empyema.