Total and differential cell counts were obtained for alternate weekly morning and evening milk from 11 Holstein cows in six different lactations. Milk from quarters suspected of mastitis were examined for presence of pathogens. Weekly cell counts for each cow showed large variations throughout lactation. The neutrophil count closely paralleled the total cell count. The average neutrophil percentage varied from 65 to 96%. No relationship was observed between cell count or type and length of lactation, age of cow, and milk yield. In addition to mastitis, unspecified stresses seemed to cause irregular sudden increases in somatic cells. Except during severe stresses, total cell counts were about 200,000 per milliliter, of which 65 to 90% were neutrophils.
1Supported in part by a research grant (A-4423) from the National Research Council, Canada, and by the Ontario Department of Agriculture and Food.