Inhalation of organic dust in a swine confinement building induces systemic reactions, increased bronchial responsiveness and intense airway inflammation in previously unexposed, healthy subjects. These effects are self-limiting, but chronic respiratory symptoms are frequently observed in swine confinement workers. The present study was aimed at investigating organic dust-induced activation of the monocytic leukemia cell line, THP-1. Unstimulated THP-1 cells proliferate in suspension but cultivation for several days in medium with complete dust or 0.22-mu-filtered suspension, caused a subset of the THP-1 cells to adhere to the substratum. As assessed by transmission light- and indirect immunofluorescence microscopy, dust-stimulated adherent THP-1 cells adopted macrophage-like morphology and expressed vimentin. Intercellular adhesion molecule (ICAM)-1 was expressed in all dust-activated adherent cells, but only in 1% of the unstimulated cells in suspension. Sialoadhesin, a macrophage marker, was detected in dust-stimulated adherent THP-1 cells but not in the parental monocytes. Serum factors were required for the dust-induced expression of sialoadhesin, but not for adhesion to substrate or expression of ICAM-1. In addition, morphology and expression of vascular endothelial growth factor (VEGF) of dust-stimulated adherent cells equalled that of PMA-differentiated THP-1 cells, but the PMA-differentiated cells exhibited weak sialoadhesin labelling. In conclusion, exposure to organic dust from a swine confinement building activated a subset of THP-1 monocytes inducing expression of intercellular adhesion molecules, which are important in inflammation. The sustained adhesion to substrate indicates that organic dust from a swine confinement building may contain agents that prevent deactivation and detachment of the cells.