The effects of malnutrition on mucosal goblet cell mucin levels were studied in rats deprived of 50% of their daily intake, as judged by pair-fed, age-matched control animals, for 5 wk. Average daily weight gain was 0.7 g/day compared with 5.8 g in age-matched (AM) rats; final weight was 246 +/- 9 g compared with 406 +/- 4 g. Immunoassayable mucin, sucrase, protein, and DNA were assayed in mucosal scrapings from the proximal, middle, and distal segments of the small intestine in malnourished rats, AM rats, and a third group of low-weight, less mature (LM) rats. Total protein, total DNA, and protein-to-DNA ratios in malnourished rats were unchanged compared with AM control rats and often higher than levels in LM control rats. In malnourished animals, mucin concentration per milligram protein was significantly decreased below AM control animals in the upper two segments and below LM control animals in all segments. Mucin concentration per milligram DNA was significantly lower in malnourished rats than in all segments of both control groups. In contrast, sucrase activity per milligram protein or DNA was either unchanged or increased in the malnourished rats, indicating that the reduction in mucin concentration was selective and did not reflect all surface glycoproteins. Isolated mucins from malnourished and AM control rats were chemically similar, and the affinity and number of antigenic determinants were the same. Malnutrition therefore leads to an absolute decrease in intestinal mucin rather than reduced molecular antigenicity. Impaired capacity to maintain mucosal mucin content may be a factor in reducing intestinal resistance to enteric infection in malnutrition.