Antibodies to porcine circovirus (PCV) which is the smallest animal virus known so far were found in 77-95 per cent of sera from slaughter pigs gathered in Berlin and two districts of Northern Germany. About 60 per cent of these positive sera had relatively high titres similar to those in experimentally infected pigs 3-6 weeks after infection. This indicates that the animals might have become infected during the fattening period. Sera from 2-3 year old pigs from a laboratory animal breeding institution were also found positive (83 per cent) but titres were lower. Experimentally infected minipigs developed antibodies and virus was isolated from nasal swabs and from fecal samples. The animals neither showed any signs of illness nor were pathological changes noticable. The assumption that PCV is a common virus in all swine populations was strengthened by the finding of PCV antibodies in wild boars shot in the forests of the Berlin region.