Summary and Conclusions: The incidence of blood groups in cases of paralytic poliomyelitis occurring under five years of age shows no radical deviation from the normal figures. Above that age there is a distinct diminution in B and a slight increase in O. This shift in the distribution of blood groups becomes progressively more noticeable with the advance in the age at onset, coincident with the drop in general morbidity. The largest number of deaths after five years of age occurred in individuals other than B, with a predominance of A in the five to ten year period and of O in the cases over ten years. The prevalence of poliomyelitis in different parts of the world appears, with certain exceptions, to diminish in direct proportion to the increase in group B and the decrease in groups A and O among the human races. This relative racial insusceptibility seems to occur independently of climatic factors. No more than one-third of six human convalescent sera belonging to blood group A nor more than about half of thirteen sera belonging to blood groups O and AB were capable of neutralizing the virus of poliomyelitis in vitro. On the other hand six out of seven B sera were found to possess this property. These results may be useful in pointing the way to a more judicial selection of suitable donors in the collection of convalescent sera form therapeutic purposes.