1. The air of four large scarlet-fever wards was found to contain large numbers of haemolytic streptococci, most of which could be identified serologically. In one ward the serological types identified were found to correspond the types infecting patients in the ward.

2. The degree of infection of the air with haemolytic streptococci showed considerable variation during the 24 hours. During the night, infection was absent or negligible, in the morning there was a steep rise followed by a slight fall in the degree of infection during the early afternoon. This variation is probably associated with the degree of air movement in the ward produced by the ward staff in the execution of their duties.

3. Haemolytic streptococci were widespread in the ward air and the serological types identified were not confined to the neighbourhood of patients infected with a particular type.

4. Though contact with infected hands, instruments, books, toys, etc., is probably of considerable importance in the transmission of infection with Strept. pyogenes, and though droplet transmission no doubt occurs as in other infections of the respiratory tract, yet the possibility of infection via the air cannot be dismissed, especially in multiple-bed wards in which patients infected with many different serological types of haemolytic streptococci are being nursed.