Divisions of Feterinary Science and Animal Husbandry, Univers,~ty of California Field observations by the senior author suggested that incomplete milk- ing of cows affected with chronic mastitis may perhaps aggravate the infec- tion. Judging from the literature, this question has been little investigated. 1Vi/inchen, Schmidt-Hoensdorf, and Schmidt (2-4) studied the practice of stripping versus nonstripping in 50 cows artificially infected with strepto- coccic mastitis. According to them, the clinical symptoms became more intense in the animals not stripped after machine-milking, whereas with the cows that were stripped, the secretion soon assumed a normal appearance. Woodward, Hotis, and Graves (6), however, experimenting with 15 cows, of which 11 were infected with Str. agalactiae, failed to find that incomplete milking aggravated the infection. To obtain further information concern- ing the effect of incomplete milking on chronic mastitis, the experiments here reported were undertaken. The University dairy herd, which provided the animals, consisted of 60 cows, milked by machine and thoroughly stripped afterwards. Data on the extent and severity of mastitis in this herd were accumulated for three years prior to this special investigation. Though the incidence of infection with Str. agalactiae averaged 30 per cent during this time, the infected animals rarely produced a visibly abnormal milk ; for, out of a total of 419 strip-cup examinations made on the infected cows in this herd during the three-year period, visible particles were found in the foremilk in only 6.9 per cent of the tests. METHODS The cows selected for these special studies were harboring Str. agalactiae in one or more quarters, but were producing a visibly normal milk. A strip- cup test, chlorine determination, cell count, bacteriologic analysis, and pal- pation of the milked-out udder for indurations were made at weekly intervals on every quarter of each cow, beginning 3 weeks before incomplete milking and continuing throughout the experimental period.