The purpose of this study was to assess changes in dairy herd milk production and milk composition associated with changes in Salmonella contamination of bulk milk on dairy farms in southwestern Ontario. Twenty-three dairy farms that had submitted milk filters for culture from which Salmonella were isolated (cases) and 23 farms that submitted Salmonella-negative milk filters (controls) were included in the study. The rolling herd averages for milk and fat of case and control farms for the months of December 1985, December 1986 and April 1987 were compared and no significant differences were detected. Case and control farms were divided into three groups (A,B,C) on the basis of Salmonella culture results of milk filters submitted at various time periods throughout the study. Daily and monthly changes in milk production and composition parameters that reflected the time periods of milk filter culture were compared. The following unconditional associations between a changing Salmonella infection status on dairy farms and changes in milk production or composition variables were significant (p less than or equal to 0.05): group A: case farms had higher plate loop counts than control farms; group B: case farms had younger cows than control farms; group C: case farms had cows with longer average days in lactation than control farms. After analytical control of confounding variables, the disappearance of Salmonella from bulk milk supplies of dairy farms was associated with a decrease in percent fat and in somatic cell count.