Abstract Eight serologically distinct strains of TNV were differentiated among 11 isolates. Separation of the strains into A and D serotypes was impracticable because of the variability in the apparent relative serological relationships in comparative tests with different antisera. Three serologically distinct strains of SV were demonstrated. Two of them, SV-A and SV-B, were activated by seven of the TNV isolates and the third, SV-C, was activated only by the four other TNV isolates which did not activate SV-A or SV-B. Ability to activate the SV isolates was not correlated with apparent serological relationships of the TNV isolates. Several of the TNV isolates were free from SV when received, but all isolates were capable of activating at least one of the serotypes of SV. Some isolates could be separated from SV relatively easily; in contrast, we have never obtained some other isolates free of SV despite repeated attempts. Thus it appears that TNV isolates differ with regards to their affinity for SV.