The intramuscular administration of parental blood serum for the prophylaxis of contagious diseases has been a routine procedure in the Morrisania City Hospital since 1934. Our experience with this method up to 1937 was the subject of a previous communication.1 The following considerations have prompted this report. Firstly, we wish to set forth additional evidence of the value of human serum as a prophylactic measure. Secondly, we desire to call attention to the simplification of the procedure since the establishment of a plasma bank. Thirdly, we feel that a detailed discussion of the technic of the procedure will be of benefit. Fourthly, we wish to attempt to explain the failure to protect the children in some hospitals where serum or placental extract was employed. Lastly, we wish to supplement our previous observations on contagion by a report on the prophylactic value of serum during an epidemic of measles and