Serological surveys, virological and bacteriological investigations were carried out on large populations of lambs and calves kept in intensive fattening units on Hungarian large-scale farms, in order to study the occurrence of viruses and bacteria involved in the respiratory disease complex. The investigations were carried out in stocks where 5-20% of the young animals succumbed to acute or chronic pneumonia every year. Serological surveys revealed that seropositivity to bovine adenovirus type 2 (BAV-2) and parainfluenza type 3 (PI-3) were the most widespread viral infections both in cattle and sheep (BAV-2: 56% and 88%, PI-3: 76% and 41%, respectively). Antibodies against infectious bovine rhinotracheitis virus (BHV-1) were also found in a high proportion (45%) of sera of 3-8 month old calves, while in 2-6 month old lambs ovine adenovirus 1 (OAV-1) was the third most frequent virus (22%). Bacteriological investigations demonstrated secondary infections caused by different Pasteurella haemolytica and Pasteurella multocida serotypes in most cases. In cattle herds P. multocida A proved to be the predominant bacterium: it was isolated in each herd examined, while P. haemolytica A1 strains were isolated in 7 and Haemophilus somnus in 2 out of 13 herds. In sheep flocks P. haemolytica was the most frequent species isolated, 10 serotypes were cultured from pneumonic sheep but serotypes A2, A1 and A8 were the dominant ones.