Exposures of South African mine workers to Legionella pneumophila and Chlamydia pneumoniae were studied over a period of six months. Seroprevalence data were compared with data from a group of factory workers. Antibodies to L. pneumophila serogroups 1-4 were demonstrated in 36% of the mine workers and 10% of the factory workers (p < 0.0001). Seroconversion occurred in 18% of the mine workers during the study period. Antibodies to C. pneumoniae were present in 66% of the mine workers and 22% of the factory workers (p < 0.0001). Seroconversion was demonstrated in 17% of the mine workers during the six months of follow-up. A significant association between the presence of respiratory symptoms and seroconversion was demonstrated (p < 0.025). The data suggest that mine workers are at increased risk for infections with L. pneumophila and C. pneumoniae. Legionella pneumophila; Chlamydia pneumoniae; mine workers; factory workers; risk factors; seroprevalence; seroconversion