1. A series of 30 normal adult human sera collected from metropolitan individuals and consisting of 10 O, 10 A and 10 B sera, were tested for neutralizing power against poliomyelitis virus. The following incidence of neutralization was obtained with the different blood groups: group O = 60 per cent, group A = 30 per cent and group B = 80 per cent. These percentages agree closely with the corresponding figures previously reported for 26 convalescent sera.

  2. The titer of three lots of pooled neutralizing O, A and B sera was determined by quantitative titration. The highest dilution capable of completely inactivating the virus was with the O sera 1:20 and with the A sera 1:5 while the B sera neutralized up to a 1:60 dilution. Similar discrepancies in virucidal titer were obtained with Wassermann samples pooled according to blood group.

  3. Three adult Eskimo sera and twelve adult Chinese sera were tested for virucidal property. All sera neutralized the virus of poliomyelitis, excepting one Chinese serum of blood group A.

  4. In view of the wide quantitative and qualitative variations in neutralizing power among normal adult human sera, depending as it seems in a large measure on the blood group of the individual, the virucidal test must be considered basically unsound as a means to determine previous exposure to the virus in epidemiological studies.

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