It has been shown by a number of studies that chronic alcohol abuse is associated with an increased incidence of infections. The mechanism of this increased susceptibility to infectious agents is multifactorial and certainly includes abuse of other drugs, smoking, malnutrition and other factors. Recently, it has become apparent that consumption of alcohol by human beings and experimental animals is associated with changes in the immune system. The increased incidence of tuberculosis and opportunistic bacterial and fungal infections described in alcoholics has led to the suggestion that alcohol affects the cell-mediated arm of the immune response. The present report will review the data that shows the effects of alcohol on the immune system. We will also review the data from our laboratory and other laboratories that show the effects of alcohol on host defense mechanisms to infectious agents. Where possible, mechanisms of increased susceptibility will be discussed or postulated.