Leukocytes from mammary secretions in dairy cows were collected during the nonlactating and postpartum periods. Differential cell counts, viability and activity of peroxidase, N-acetyl-beta-D-glucosaminidase (NAGase, beta-glucuronidase and alpha-mannosidase in cells were determined. Cell viability (trypan blue exclusion) was 75-80% during most of the nonlactating period, but declined to 45-50% by parturition. Polymorphonuclear neutrophils (PMN) predominated during the first week of involution, after which macrophages were the predominant cell type. Peroxidase activity in leukocytes from mammary secretions was high in early involution, probably reflecting the predominant peroxidase-containing PMN. Peroxidase activity declined through the remaining nonlactating and postpartum periods. The activity of NAGase was variable in early involution, then increased to a peak during the mid-nonlactating period, before declining prior to parturition. Activity of beta-glucuronidase generally was unchanged during the nonlactating period, although NAGase and beta-glucuronidase activities were significantly and positively correlated throughout the period studied. Activity of alpha-mannosidase changed in a manner similar to peroxidase activity.