1. In two hospitals, post-operative staphylococcal wound sepsis occurred with equal frequency in patients who were and in those who were not nasal carriers of Staph. aureus.

2. Nasal carriage of Staph. aureus was not always associated with skin carriage at the site of operation, but even patients who were skin carriers suffered no more sepsis than non-carriers.

3. Self-infection was a less frequent cause of sepsis than was cross-infection. Other investigations of this type have revealed conflicting findings on the frequency of self-infection. These may be due to differences in the epidemiological properties of staphylococci infesting the hospitals; to different methods of preoperative skin preparation; and to the different types of operation concerned.

4. In one of the hospitals, 31% of probable operating-room infections were with organisms similar to those carried by some member of the surgical team. In infections thought to have occurred in the ward, only 10% showed this similarity. In 11 of 35 presumed operating-room infections, the source may have been a member of the surgical team.

For help and co-operation we are grateful to Mr G. A. Mason, Senior Surgeon, Newcastle Regional Thoracic Surgery Service; and Dr Joan Millar, Miss G. Jones, Miss E. Smith, Mrs Jean Best, Miss Winifred Mahon, Dr K. Hodgkin, Dr W. Stewart, Mr S. Griffin, Mr W. C. Barnsley, Mr R. Dobson, Mr W. H. Bound, and many nurses and other hospital workers. Our especial thanks are due to Professor R. E. O. Williams, who phage-typed many of the staphylococci, and then taught us to do this for ourselves.