Two vaccinia viruses isolated from patients with vaccinial complications (vaccinial ulcer, postvaccinial seizures) showed qualitative differences from the original parental strain. After intradermal injection of the viruses into the rabbit marked necroses developed, which the original strains did not produce. While the parental virus did not grow on the chorioallantoic membrane at 41 degrees C after 2 days incubation, the vaccinia variant produced typical lesions at that temperature. Also the yield of infectious virus on various cell systems was 1--2.5 logs higher for the virus than for the original vaccine strain. With the plaque technique differences were seen in the appearance and size of plaques between the variant and the parental vaccinia strain. These results indicate that virus of an increased pathogenicity could be isolated from the patients and this might be causally connected with the postvaccinial complications from which they were suffering.