Relationship between milk production and milk composition was studied through comparisons of udder halves within cow. Cows were milked by milking unit for separate quarters of udder. Six trials had six cows per trial. Trial length was 3 d, and milkings were at 12-h intervals. Foremilk samples were taken aseptically for bacterial analysis. Milk weights by quarter were recorded, and samples by quarter were analyzed for concentrations of lactose, somatic cells, and chloride. Milk cell differential counts and N-acetyl-B-D-glucosaminidase activity also were determined. Eighty-four percent of quarter milk samples contained less than 400,000 cells/ml. Differences between right and left udder halves with respect to all measurements were computed. For halves of udders within-cow correlation coefficients for differences between production and log(base 2) somatic cell count, lactose, chloride, bacterial presence, neutrophil percent, lymphocyte percent, macrophage percent, and N-acetyl-B-D-glucosaminidase activity were -.16, .23, -.31, .09, .12, .01, -.14, and -.41. Regression coefficients of milk production (kg) on somatic cell count log(base 2) cells per milliliter, lactose (%), chloride (mg/100 ml), and N-acetyl-B-D-glucosaminidase (nmol/min per ml) were -.12, .57, -.05, and -.46. From negative correlations between production and concentrations of chloride, somatic cells, and N-acetyl-B-D-glucosaminidase activity, differences between udder halves in production may be related to changes of the blood-milk barrier, leukocyte diapedesis, and loss of integrity of secretory cells.