An analysis of the incidence of colds in volunteers, exposed experimentally to various rhinovirus and influenza virus challenges, confirmed an earlier report that assessments of virus shedding show a higher degree of infection by rhinovirus in Introverts than they do in Extroverts. The influenza trials however did not show such an effect. In addition, people with higher scores on a widely used self-report inventory of mild psychoneurotic symptoms tended to show more evidence of increased nasal secretion after infection by either type of virus. The specific scale showing the latter effect most clearly was that of Obsessional Symptoms. Taking account of these psychological factors greatly increased the sensitivity of a study of the beneficial effects of interferon.