Human neutrophil lipocalin (HNL) is a recently identified protein from human neutrophil granules. The concentrations of HNL in the circulation were measured, in a group of patients with acute infections, using a radioimmunoassay. The concentrations of HNL in patients infected by viruses and bacteria were 93.78 +/- 45.30 micrograms l-1 (SD), 404.14 +/- 355.02 micrograms l-1 (SD) in serum, and 47.81 +/- 18.18 micrograms l-1 (SD), 145.46 +/- 194.32 micrograms l-1 (SD) in plasma, respectively. The differences between the two patient groups were highly significant. There was a significant correlation between serum HNL and plasma HNL levels in bacterial infections (r = 0.73, p < 0.0001). The HNL serum levels also correlated with those of C-reactive protein (CRP) (r = 0.59, p < 0.0001). Determination of HNL in serum was more specific and sensitive than CRP in the distinction between viral and bacterial infections. At a cut-off of 155 micrograms l-1 (HNL in serum), the positive and negative predictive values for the diagnosis of bacterial infections were 92 and 96%, respectively, which were superior to the optimal predictive values of CRP. Thus, the determination of HNL level is useful in the diagnosis of acute bacterial infections.