Over the past 7 years we have enrolled several hundred families in a study of the prophylaxis of atopic allergy. Initially, control and study patients were at different institutions, but more recently a true double-blind random assignment to control or study group has been instituted. Nevertheless, all of the infants from these highly atopic families can be utilized to obtain the data reported; that is, the rate of change of IgE levels, the appearance of food sensitization (food-specific IgE), the signs and symptoms of atopic allergy diseases and the changes in cow's milk-specific IgG antibody during the first 2 years of life. Some aspects of the first three of these parameters have been presented in the eight references to this paper, but only preliminary data on the use of IgG anti-cow's milk antibody as a measure of compliance has been available. In this paper an analysis is presented of the various patterns of IgG antibody levels in the first 60 infants of the over 200 who have been studied to date. Both the age of onset and the subsequent changes in the foregoing immunologic measurements provide fundamental data with which to measure the success rate of any prophylactic or therapeutic regimen and may improve our capacity to predict the future course of infants and children with a strong familial tendency to allergy.