For some years it has been recognized that in a certain percentage of infants there occurs in the first few days of life a rise in temperature. This fever has not been accounted for satisfactorily, so far, by any explanation which we regard as adequate to explain temperatures at other periods of life. This rise of temperature has been thought to occur with peculiar frequency at the time when the weight is at the lowest point and also in those infants whose weight loss is most marked. It, therefore, is most commonly seen on the third, fourth and fifth day. It is practically always transient in character and is accompanied by almost no disturbance of the general clinical findings. Consequently, it offers an example of a febrile condition apparently very simple in nature. It was thought that a very careful clinical study of this condition might throw some light on