Four serotype 1 Marek's disease virus (MDV) strains were isolated from Marek's disease (MD) outbreaks in Germany and Spain and their virulence was tested in SPF White Leghorn chickens. Two of the isolates from Germany (MR48/1-2 and /2-8) induced early mortality from cytolytic MD in 25% of the chickens whereas one isolate from Germany (MR48/3-16) caused early mortality in 58% of the inoculated chickens. Histological examination showed severe depletion of the lymphoid tissue of the bursa of Fabricius and the thymus, consistent with the cytolytic phase of MD. AH surviving chickens later developed acute MD in the form of multiple visceral tumours. Inoculation of SPF chickens with various doses of the isolate MR36 8/1C (Spain) caused early mortality in up to 69% of the chickens and acute MD in 92 to 100% of the chickens surviving the cytolytic phase of the disease.