Air samples for determination of the concentration of Gram-negative bacteria, dust and endotoxin were collected at 10 sites in 2 large pulp and paper mills (paper factories) located in northern Poland, of which one (plant "A") was an older type facility while the other (plant "B") was a modern, fully automated factory with an effective ventilatory system. In both factories paper was produced from wood chips derived mostly from Scots pine. The concentrations of Gram-negative bacteria in the air of examined factories were within a range of 11.0-310.0 cfu/m³, being greatest in the old type factory "A" at the initial stages of production cycle comprising handling of chips and pulp production. The mean value for these sites (246.9 cfu/m³) was significantly greater (t-test, p<0.01) compared to final stages of paper production in the same factory (mean 32.1 cfu/m³) and to corresponding stages of chip handling in the modern "B" factory (mean 94.4 cfu/m³). The values of the respirable fraction of airborne Gram-negative flora were at most sites within a range of 40.0-56.9%. The species of the family Enterobacteriaceacae, mostly belonging to the genera Enterobacter, Pantoea, Rahnella and Klebsiella, distinctly prevailed in the air of the examined factories. Altogether, 19 species or genera of Gram-negative bacteria were identified in the collected air samples, out of these 9 were reported as having allergenic, immunotoxic and/or infectious properties. The concentration of dust in the air of paper factories ranged from 0.13-3.9 mg/m³ and never exceeded the safe level. The concentration of bacterial endotoxin in the air of paper factories varied within a fairly wide range of 0.0042-2.5 µg/m³. At 4 sites associated with initial chip handling and pulp production large concentrations of airborne endotoxin between 0.2-2.5 µg/m³ were found, significantly exceeding suggested safe levels. In conclusion, despite Gram-negative bacteria occur in the air of paper mills in relatively low concentrations which never exceeded the value of 1,000 cfu/m³ proposed as safe level, they may exert adverse effects on exposed workers, as evidenced by high concentrations of airborne endotoxin and the presence of numerous potentially pathogenic species. Thus, these microorganisms pose a potential risk of respiratory disease for the workers of pulp and paper mills, in particular for those engaged in handling of wood chips and production of pulp.