1. A comparative study of the mucin virulence of smooth, converted rough, and respiratory strains of H. influenzae was carried out in mice.
2. The smooth strains were found to be significantly more virulent than either the converted rough or respiratory strains.
3. The converted rough and respiratory strains were found to be approximately equal in virulence.
4. A diminution in virulence was observed in those smooth strains which had dissociated spontaneously either partially or completely.