Forty-two Clostridium perfringens type A strains isolated from cases of diarrhoea in pigs were tested for their ability to sporulate and produce enterotoxin in three different sporulation media. Enterotoxin was produced by 11 of the 42 C perfringens type A isolates (26.2 per cent). Thirteen isolates (30.9 per cent) produced spores at a frequency of 10 per cent or more. Spore production was recorded in 24 (57.1 per cent) of the isolates. The titres of enterotoxin produced by the isolates ranged from 1:2 to 1:64. The enterotoxin produced was compared with that produced by a reference strain and found to be identical. Ninety-eight of 106 sow sera from four different farms were found to possess antibodies to C perfringens type A enterotoxin with titres ranging from 1:2 to 1:64. Spores of C perfringens type A were detected in pig faeces and intestinal contents in 20 of 23 cases of enteritis at levels of up to 5 x 10(6) cells/g of faeces. Smaller numbers of spores, up to 2 x 10(4)/g were present in five of 10 samples from non-diarrhoeic pigs. Enterotoxin was demonstrated by Vero cell assay in five of the 23 samples from diarrhoeic pigs but in none of the 10 samples from non-diarrhoeic animals. It was clear from these studies that C perfringens type A strains in pigs could sporulate and produce enterotoxin in vitro and in vivo and that enteritis might be associated with sporulating organisms in vivo.