Allergenicity and antigenicity of various commercially available cow milk hydrolysates intended for infant feeding were analysed in 45 children with cow milk allergy. The hydrolysates included the whey hydrolysates Beba HA® (Good Start HA®) and Profylac®, and the casein hydrolysates Alimentum® and Nutramigen®. Positive skin prick tests were recorded against Beba HA in 10 of 41 tested children (24%), against Profylac® in 5/34 (15%) and in one each (2.5%) against Alimentum and Nutramigen. Double-blind placebo-controlled oral challenge tests were performed in 11 children with cow milk allergy using Alimentum, cow milk (positive control) and their regular well-tolerated formula (Nutramigen or soy) used as negative control. One child reacted to Alimentum. This patient was the only one with circulating antibodies against the product, as indicated by a positive RAST. High density SDS-PAGE electrophoresis showed that Beba HA contained a number of unresolved proteins, and non-degraded or partially degraded whey proteins in the range of 5–20 kD. Profylac contained strongly stained protein material in the low molecular weight region 1–10 kD. No protein bands could be identified in the casein-based hydrolysates. Residual antigenicity was tested by measuring the content of betalactoglobulin in the hydrolysates. Three of the hydrolysates contained < 0.06 μg/g dry weight, while the concentration in Beba HA was 200 μg/g dry weight. Positive RAST against Beba HA was detected in 11/45 sera (24%) compared to 7–13% against the other hydrolysates. RAST inhibition with the hydrolysates using cow milk discs was very low for all of them. Using dot immuno-binding assay a weak IgE binding with Alimentum was detected in 4 sera, Beba HA and Profylac in each 2 sera and with Nutramigen in one. The data taken together show that all 4 tested hydrolysates retain some allergenicity. There were differences between the products, one of the whey hydrolysates being substantially more allergenic and antigenic than the other tested formulas. The casein hydrolysate Alimentum showed few reactions in vivo and in vitro in this selected group of children but one child reacted when challenged with Alimentum, indicating that there is a risk for general reactions when using any hydrolysed product in subjects allergic to cow milk.