Antibiotic treatment eliminated organisms from 61% of staphylococcal and from 80% of streptococcal udder infections. In 84% of quarters from which organisms were eliminated the cell counts of the milk were reduced, about half of them to a normal and about one-third to a late-lactation type of cell count in which the majority of the cells are epithelial cells. The remainder became reinfected before there was time for the cell counts to return to normal. It was found that it may take as long as 5 weeks for the cell counts to return to normal after elimination of mastitis organisms, but in quarters in which the cell count eventually fell there was a decrease in the percentage of polymorphs in the milk to less than 30% one week after treatment.