The bacterial single cell protein (BSCP), BioProtein, is dried bacterial mass derived from fermentation of the gram negative bacteria Methylococcus capsulatus, used for animal and fish feed. Workers in this industry suffer frequently from pulmonary and systemic symptoms which may be induced by an inflammatory reaction. The aim of the present study was to examine the effect of BSCP on inflammation in vitro as evaluated by complement activation and cytokine production. Human serum was incubated with BSCP and complement activation products specific for all pathways were detected by enzyme-linked immunosorbent assay (ELISA). Human whole blood anti-coagulated with lepirudin was incubated with BSCP and a panel of 27 biological mediators was measured using multiplex technology. BSCP induced a dose-dependent complement activation as revealed by a pronounced increase in alternative and terminal pathway activation (fivefold and 20-fold, respectively) at doses from 1 microg BSCP/ml serum and a similar, but less extensive (two- to fourfold) increase in activation of the lectin and classical pathways at doses from 100 and 1000 microg BSCP/ml serum, respectively. Similarly, BSCP induced a dose-dependent production of a number of cytokines, chemokines and growth factors in human whole blood. At doses as low as 0 x 05-0 x 5 microg BSCP/ml blood a substantial increase was seen for tumour necrosis factor (TNF)-alpha, interleukin (IL)-1-beta, IL-6, interferon (IFN)-gamma, IL-8, monocyte chemoattractant protein (MCP)-1, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, IL-4, IL-9, IL-17, IL-1Ra, granulocyte-colony-stimulating factor (G-CSF) and vascular endothelial growth factor (VEGF). Thus, BSCP induced a substantial activation of all three initial complement pathways as well as a pronounced cytokine response in vitro, indicating a potent inflammatory property of this agent.