During acute infection, the concentration of fibrinogen (Fb) increases in the plasma. It is controversial whether this is beneficial for the host. In this study, using an in vitro assay, the ntracellular killing of S. aureus, S. pneumoniae, E. aerogenes and C. albicans by human granulocytes was investigated in whole plasma including different concentration of Fb. Granulocytes were allowed to phagocytose opsonised microorganisms for a short time (5 min), the noningested organisms were removed by a series of differential centrifu-gatinon and the phagocytes containing the ingested organisms were reincubated in whole plasma with different concentration of Fb. After various periods of reincubation (at 0, 30, 60, and 120 min) the number of viable microorganisms was determined by a microbiologi¬cal method. Regardless of the type of microorganism, groups with 2 and 8 mg/ml of Fb had markedly enhanced neutrophil microbicidal activity as compared to group without added Fb (p