Some infants intolerant to cow's milk protein (CMP) are often also intolerant to other food proteins including soy protein (SP). The effect of CMP and SP in infants recovering from diarrhoeal disease was studied in 22 infants who were maintained on an hypo-allergenic formula for 4-6 weeks. The infants were then challenged successively, initially with SP, followed 24 h later with CMP and then rechallenged with SP 24 h after CMP provocation. Three groups were recognized on the basis of clinical symptoms and mucosal changes following SP challenge. Group 1 comprised four infants who developed clinical and histological reactions on SP challenge. The subsequent CMP challenge, 24 h after the initial SP challenge, resulted in clinical symptoms in three of the four infants, and they developed increased mucosal injury. Rechallenge with SP in the three infants caused development of severe clinical symptoms. Group 2 comprised 12 infants who developed histological reaction but had no clinical symptoms to initial SP challenge. The subsequent CMP challenge caused further progression in mucosal pathology in 11 of the 12 infants and six also had associated clinical symptoms. Rechallenge with SP in the latter six infants resulted in development of clinical symptoms in three and tolerance to SP in three infants. Group 3 comprised six infants who tolerated SP and CMP but one of these infants developed mild histological changes to CMP. The progression of mucosal injury following SP and CMP challenge was associated with a significant decrease in mucosal disaccharidases, alkaline phosphatase levels and presence of reducing sugar in the stools. The 1 h blood xylose level continued to decrease significantly following the pre-SP, post-SP, and post-CMP challenge. It appears that the small bowel mucosa of young infants recovering from diarrhoeal disease remains sensitive not only to CMP but also to SP. The feeding of these proteins in rapid successive sequence to infants with mucosal damage might result in further progression of the mucosal injury. Thus, the exclusion for a variable period of time of antigenic food proteins like CMP and SP from the diet of young infants recovering from diarrhoea might reduce the risk of inducing mucosal sensitivity to these proteins in susceptible infants.