Polymorphonuclear leukocytes were isolated from 12 patients with acute bacterial infections and the ability of the chemoattractant formylmethionyl-leucyl-phenylalanine (FMLP) to bind and induce a metabolic response in these cells was investigated. Cells isolated from the patients showed a significantly increased metabolic response in a luminol enhanced chemiluminescence assay compared to cells, isolated and analyzed in parallel, from healthy controls i.e. the patient cells were primed. The primed state was, as calculated by Scatchard analysis, accompanied by a significantly increased number of FMLP receptors exposed on the cell surface while the receptor binding affinity remained unchanged. There was, however, no correlation between the degree of priming and the degree of receptor upregulation. Furthermore, it was found that stimulation also with phorbol myristate acetate (PMA), a substance lacking specific cell surface receptors on the PMNL, gave rise to an increased metabolic response in the primed cells. These results indicate that the priming activity induced by a bacterial infection can only partly be explained by receptor modulation and that other mechanisms must also be considered. This study was approved by the Ethics Committee of the Medical Faculty, University of Linköping.