THE common catarrhal cold is responsible for much sickness disability, and preventive vaccines containing a mixture of the predominant microbes present in the secretions have been employed, the microbes being bacteria such as M. catarrhalis, B. hofmanni, Friedlander's bacillus, Pneumococcus, Staphylo-coccus and Streptococcus. Trials of such a vaccine in the past, some of them on a large scale, have been disappointing, occasionally seeming to be useful in individual cases, but not significantly so in any large groups. A further test by the Chief Medical Officer of the Post Office during the last three years upon a large scale has, it is announced, similarly been disappointing. Volunteers were invited from large towns, and some hundreds of them were inoculated in the autumn of the three years 1933–35, and their sickness rates over periods antedating the inoculations and during the treatment were compared. Comparisons were also made with large control groups in each place of uninoculated workers. About ten per cent of the volunteers—less than half the original number—who persisted throughout the three years of the experiment showed some improvement, but taken as a whole the results were not encouraging, for there was little reduction of sickness among the whole group compared with the control groups or with their previous record. The use of anti-catarrhal vaccine as a large-scale routine measure in the future is, therefore, not considered justifiable. The negative character of the results of this experiment is not unexpected, as the common cold is now regarded as being a virus disease, the bacterial organisms associated with it being of the nature of secondary invaders.