There are numerous alternatives to cows' milk formula for allergic children. We have investigated the allergenicity of several of these using RAST and RAST inhibition on serum from 16 patients with a known history of cows' milk protein intolerance (CMPI) and 16 atopic controls. A RAST grade of > or = 3 for cows' milk was present in all those with CMPI, whilst all the controls gave RAST of < or = 1. Modified cows' milk formula, goats' infant formula, sheep and goats' milk produced similar results to cows' milk. Only two patients had RAST > or = 3 for soya milk and the soy/beef hydrolysate gave positive results in only three patients. One had positive RAST to Nutramigen and two to Pregestimil. Of the whey hydrolysates investigated, Pepti-junior gave seven positive RASTs whilst we were unable to bind Alfare to the sepharose in sufficient quantities to interpret the results which were negative in all cases. RAST inhibition data on pooled sera from the same patients agreed with the RAST results. The inhibition curves showed high inhibition with goats', sheep, modified cows' milk formula and the casein formula, AL110 (50%). Soy and soy/beef hydrolysate showed a much lower inhibition pattern. Casein hydrolysates showed low inhibition while the whey hydrolysate produced higher inhibition. We have shown that despite claims of low allergenicity, some of these alternative formulae are antigenically recognized in vitro by some cows' milk intolerant patients.