The aim of the project was to investigate whether there were diminished health effects in swine farm workers during summer compared with winter, as seasonal differences in concentrations of bioaerosols have been reported. Twenty-four workers were visited once during each season. Before and after a work shift, they underwent lung function testing and blood sampling. During work, they wore personal air sampling equipment. The mean endotoxin exposure of the workers was highest during winter (25,690 vs. 6,553 EU/m³; p = 0.004). Although exposures to endotoxin and CO₂ varied between the seasons, no differences in lung function were found between them. White blood cell concentration increased over the work shift from 5.74-6.82 in winter (p<0.0001) and from 5.80-6.38 in summer (p=0.014). These increases differed between the two seasons (p=0.032). Plasma tumour necrosis factor concentrations fell over the work shift only during winter (1.34-1.24 pg/ml (p=0.03) (p=0.014 for the difference between seasons). Plasma interleukin-6 increased over the work shift independently of season (p=0.0006). The study supported our hypothesis of adverse effects on lung function and immune system, but less so during summer than during winter among Quebec swine farm workers.