In the United States, 1.2 million workers are exposed to metalworking fluids. During operations, aerosols are produced and airborne contaminants can be inhaled. Although biocides are used to control the bacterial content of metalworking fluids, they can create health-related problems, and their efficiency remains to be proved. The objectives of this project were (1) to verify whether rigorous cleaning according to a standard protocol could reduce microbial contamination and (2) whether the use of biocides with different spectra could reduce the bacterial population. Four similar machines producing similar components were evaluated; a specific treatment was applied to each machine. The machine used as a control (1) was thoroughly cleaned prior to sampling, (2) did not undergo any major cleaning afterward, and (3) was operated without the use of any biocide. A major cleaning is a protocol described and recommended by the fluid manufacturer and was performed on the three other machines, two of which were subsequently treated with biocides weekly. Fluid samples from the four lathes were collected weekly during a 6-month period, and total bacterial and cultivable Gram-negative bacteria were analyzed for each sample. Major cleaning of the machines (120-4) did not significantly reduce the concentration of bacteria in the cutting fluids when compared with the control machine (120-3), which had not undergone major cleaning. The concentrations of total bacteria were in the 10(6) CFU/mL range for these two lathes; however, a reduction in the total number of fluid changes was observed for this machine. Bacterial flora in the cutting fluids was significantly controlled with the use of biocides. Bacteria concentrations were in the 10(3)-10(5) CFU/mL range for the lathes with the use of biocides. Since thorough cleaning is insufficient and biocides are recognized as being responsible for some worker health problems, other avenues for controlling bacterial flora in cutting fluids should be evaluated to reduce worker exposure to their bacterial contaminants.