The study evaluated the effects of triazine use and machine enclosure on workers' exposure to bacteria, endotoxins, and formaldehyde in the use of metalworking fluids (MWF). Concentrations of triazine and bacterial contaminants were monitored in bulk samples of MWF from two machines in one workplace. One of the machines was used normally; triazine was added to the other when needed. Air sampling was used to estimate workers' exposure to endotoxins at 18 workplaces near enclosed and open machines. Concentrations of triazine in MWF and formaldehyde in the air were measured. Some recirculating local exhaust ventilation systems were also tested. The endotoxin and bacteria concentrations in the biocide test of MWF rapidly increased when the biocide levels decreased below 500 ppm. Airborne concentrations of endotoxins were substantially lower near enclosed machines than near open ones. Concentrations of airborne formaldehyde were below the Finnish occupational exposure limit at the existing levels of triazine in MWF. Concentrations of triazine in MWF correlated well with those of formaldehyde in the air near the machines (correlation coefficient r=.69). The results showed that the triazine levels in MWF should continuously be kept high enough (>500 ppm) to prevent workers' exposure to endotoxins and bacteria. Overdosing with triazine, however, should be avoided, so that the levels of airborne formaldehyde remain low. Triazine levels in MWF can be followed by the method described in this article. Workers' exposure to contaminants of MWF can be reduced substantially by enclosing the machines and equipping them with local exhausts.