In order to obtain data about the significance of enterotoxigenic E. coli (ETEC), and of the different enteric viruses in the aetiology of porcine postweaning diarrhoea, bacteriological, electron microscopic and ELISA studies were made on dead and live pigs. E. coli from the small intestine of diarrhoeal weaned pigs that died were tested for serogroups, pili (fimbriae) and toxin-geno-type. The 108 haemolytic E. coli representing 14 farms and 42 pigs were typed as follows: 0149:K88+ETEC (56.5%), OX:K88+ETEC (21.3%), O141:F18ac+ ETEC (4.6%), O147:F18ac+ETEC (3.7%), OX and O157:F18ac+ETEC (9.3%), verotoxigenic O141:F18ac (2%). In another study, when faecal samples of 92 live diarrhoeal weaned pigs (representing 19 farms) were tested, rotaviruses (18.6%), porcine epidemic diarrhoea virus (PED) (5.5%) and calici-like viruses (5.5%), and adenovirus (two pigs) were detected, besides K88+ETEC (12%) and K99+ or 987P+ETEC (one of each). Combined infections were detected in 9% of the samples. Sequential studies of diarrhoeal and nondiarrhoeal weaned and unweaned pigs indicated that PED virus and group A rotavirus were related to diarrhoea but adeno- or calici-like virus were not. It was concluded that K88+ETEC was the overwhelming aetiologic agent of porcine postweaning diarrhoea in Hungary, but F18ac+ETEC, group A rotavirus and PED virus were also significant.