1. Among infants attending Child Welfare Clinics there was a steady decline in the incidence of nasal carriage of Staph. aureus over the first year of life.
2. Infants born in hospital showed a much higher incidence of Staph. aureus in the first 2 months of life than infants born at home. A similar difference was also very distinct in the later part of the first year of life.
3. Mothers delivered 2 weeks to 2 months previously had a higher incidence of nasal carriage of Staph. aureus when delivered in hospital than when delivered at home. After this interval the incidence was similar in all groups of mothers throughout the rest of the year.
4. In the first few months of life there was a tendency for carriage in infants to be associated with carriage in the mother, but towards the end of the year there was no such relationship. This association appeared most clearly in infants born at home and in infants 2–5 months old born in hospital F.
5. A very high proportion (25 out of 30) of the strains tested from infants born in hospital F less than 5 months previously were penicillin-resistant. Infants born at home had a much lower incidence of resistant strains.
6. A high proportion of strains of Staph. aureus isolated from mothers delivered in hospital F were resistant throughout the first year. Strains from mothers delivered in hospital C or at home showed a much lower proportion of resistant strains.
7. Phage-typing showed a great variety of types, but those found among the penicillin-resistant strains were rather less varied than among the sensitive strains. When Staph. aureus was found both in infant and mother, although the same phage type was commonly present in both, in 16 out of 35 couples different types were found.
8. Investigation in maternity hospitals C and F showed a high proportion of penicillin-resistant strains of Staph. aureus among the nurses and infants and in the dust of the nurseries, but not among the mothers. There was no important difference between the two hospitals as regards the incidence of resistant strains isolated from nurses and infants in spite of the differences found in babies from the two hospitals when swabbed at the clinics.
9. The significance of these results is discussed.