The purpose of this study was to determine whether a new ultrafiltrated whey hydrolysate infant formula, Profylac, could be administered safely to children with cow milk protein allergy/intolerance. Profylac has a stated molecular weight of < 8 kD and at least 30,000 times reduced antigenicity which is controlled by a combination of ELISA-techniques and immunochemical methods. The study comprised 66 children with cow milk protein allergy/intolerance diagnosed by controlled elimination/challenge procedures. The children were aged 1 month-14.5 years, median 1 11/12 years and 15 were below 1 year. Thirty-five of these children had proven IgE-mediated reactions (cow milk protein allergy). Sixty-one of the children had at least two different symptoms and 31 had concomitant allergies to other foods and/or inhalants. All 66 children underwent and tolerated open, controlled challenges with Profylac. A total of 64 children continued having Profylac daily for at least 3 months and 58 for at least 6 months after challenge. Nine of the children older than 1 year did not like the taste and only had Profylac in minor amounts. No side effects were registered. Fifteen of the infants were below 1 year of age, and this group was compared with an age matched group of 16 infants challenged with and fed an extensively hydrolysed casein hydrolysate, Nutramigen. All the infants in these two groups accepted and tolerated Profylac and Nutramigen, respectively, and no side effects were registered. Among the 35 patients with IgE-mediated reactions 6% (2/35) had positive skin prick tests and 11% (3/28) had specific IgE class 2 against Profylac, 2 of the latter before intake of Profylac.(ABSTRACT TRUNCATED AT 250 WORDS)