Undegraded carrageenan was administered orally to pigs at levels of 0 (control), 50, 200 and 500 mg/kg body weight/day for 12 wk. No effects attributable to carrageenan treatment were seen with respect to behaviour, body-weight gain, feed utilization, haematology, blood chemistry, urine analysis or organ weights. The treatment resulted, however, in changes in the intestinal flora: the total counts of aerobic bacteria were decreased in the colon and rectum and the number of Lactobacilli was reduced in the rectum. No ulcerative colitis or erosions of the mucous membrane of caecum and colon were observed. A few focal areas with an irregular surface were observed in the colonic mucosa of some of the pigs given 200 or 500 mg carrageenan/kg. A shift was seen in the cellular infiltration pattern of the mucosa of the large intestine of pigs from all the groups given carrageenan. The slight changes in the intestinal mucosa are not considered to be a significant toxic effect.