Researchers conducted an epidemiologic investigation of an outbreak of brucellosis at a meat-packing plant, where work was interrupted by a strike. The investigation revealed that the risk of infection with brucellosis may coincide with the rate at which reactor cattle are slaughtered, a factor often overlooked in other reports which investigated outbreaks of brucellosis. The slaughter of 20 to 25 reactor cattle per day was the estimate of the critical number of reactor cattle when the risk of infection is most likely to occur. Of the 193 workers studied, 17 (8.8%) were seropositive, but only eight (4.1%) were presumptive cases. Young, newly employed workers who did not wear glasses were at greatest risk of infection.