Fifty herds with the highest annual average mastitis cell counts in England and Wales were compared with 50 herds with the lowest cell counts. The milk-quality test results of these herds for milk fat, protein and lactose contents, and total bacterial count (TBC), over the 6 winter months, October 1982 to March 1983, were examined.

The herds with high cell counts had an arithmetic mean of 2.50 × 109 cells/1 and the herds with the low cell counts had 0.117 × 109 cells/1. The milk-quality test results were: milk fat, 39.8 and 40.0 g/kg; protein, 33.2 and31.5 g/kg; lactose, 42.8 and 46.9 g/kg; and TBC, 139 and 12.0 × 106/1 for the herds with the high and low cell counts, respectively. There were highly significant differences in protein and lactose contents and in TBC. When the average monthly results were compared, these differences were statistically significant in all months. On average, lactose was 9% lower and protein 5% higher in the herds with the high cell counts when compared with the herds with the low cell counts.