Investigations at two hospitals, P and S, showed Staphylococcus aureus to be commonly present in the milk of healthy nursing mothers and the throat and intestine of their babies. In hospital P, the proportion affected in a group of eighty-two mothers and their babies was over 90%. The condition may not be uncommon in maternity hospitals, but no information of its occurrence is available from private obstetrical practice. The cocci concerned seemed to be of low virulence, for babies swallowing relatively massive doses in their mothers' milk suffered no perceptible ill effect. On the other hand, the coccus was found in the pus of the only case of breast abscess encountered, and in some minor external pustular lesions. It was present in the milk of all the cases of flushed breast examined, but its causal connexion with this condition awaits proof.

The staphylococci, in the majority of cases, seemed to occur first in the baby's throat, possibly from a focus of infection in the babies' ward, and to be transmitted from baby to baby and from baby to mother, with a return infection, in larger amount, through the mother's milk. The mother's nose and throat must be included amongst other, but less frequent, sources of the infection.

The predominating type of Staph. aureus gave a relatively strong coagulase reaction which was caused only by actively growing cocci, but it would seem that the reaction caused by this agency is less trustworthy as an index of virulence of the coccus than one in which free coagulase also operates.