Milk samples from quarters were collected from 48 Holstein cows at 1.5, 3, 21, and 35 wk postpartum and at 1 wk after drying off. Foremilk samples were obtained for bacteriological examination, somatic cell count of milk, and for assay of phagocytosis. Thirty-two cows in first lactation and 16 in third lactation produced by two criteria of sire selection (high milk versus multiple trait) were studied. In vitro phagocytosis was assayed four times on each original quarter milk sample, yielding 3,840 determinations. Variance components were large for cow-associated variation. Cow, stage X cow, and cow X sample dilution were 18.7, 20.6, and 6.0% of total variance. Phagocytosis was 9% higher in skim milk from third lactations than from first. Phagocytosis was highest in dry period samples, followed by 1.5, 35, 3, and 21-wk postpartum samples. Milk somatic cell count tended to be related more closely to phagocytosis than was current bacteriological status of the quarter. Skim milk from genetically higher producing cows was less conducive to phagocytosis than skim milk from genetically lower producing cows, possibly because of dilution.