The immunoglobulin G (IgG) concentrations of 254 raw milk samples collected from the province of British Columbia (Canada) in 1990 and 1991 ranged from 0.030 to 0.71 mg ml‐1, with mean and median values of 0.28 and 0.27 mg ml‐1respectively in 1990, and 0.25 and 0.24 mg ml‐1respectively in 1991. Although significant differences were noted in IgG contents of milks from five geographical areas and 14 dairies for each of the 2 years, there were also significant differences between milks from year to year. All of the milks showed measurable antigen‐binding activity in enzyme‐linked immunosorbent assay (ELISA) against the lipopolysaccharide (LPS) fractions of five bacteria (Shigella flexneri 1A, Escherichia coli O111:B4, Escherichia coli O128:B12, Salmonella typhimurium and Salmonella enteritidis). An immune milk obtained from vaccinated cows showed significantly higher anti‐S. enteritidis LPS activity than any of the raw milk samples. However, levels of specific anti‐LPS activity against the other four bacteria were roughly of the same order of magnitude for raw milk samples and the immune milk.