Genetic marker tests were performed on 61 strains of poliovirus isolated during the Blackburn outbreak from cases or suspected cases and their contacts. The results were correlated with the epidemiological data and good agreement was found for the serological and dextran inhibition tests. The reproductive capacity temperature marker, however, showed inhibition at 39·8° C. of strains otherwise judged to be of naturally occurring type. These strains could be differentiated from those of vaccine origin because they grew readily at 39·2° C. This feature also distinguished the strains isolated in Blackburn from those obtained in the nearby town of Hyde in a simultaneous outbreak.
Dr J. Ardley, Medical Officer of Health, Blackburn, kindly provided a key which enabled the contacts of cases to be traced; and Dr L. Robertson, Public Health Laboratory, Preston, made available his extensive records as well as the many strains isolated in his laboratory.