The use of L-tryptophan for balancing tryptophan deficient diets in animal feeding is currently impeded by the cost of feed-grade product. Recently ADM CORP. (Decatur, IL, USA) developed a method whereby liquid L-tryptophan and liquid L-lysine-HCl could be blended before drying. The final product contains at least 15% L-tryptophan and 70% L-lysine-HCl (Trademark Tryptosine). Experiments have been conducted with pigs and broilers to evaluate the efficiency of tryptophan from Tryptosine in comparison with feed grade L-tryptophan. Results indicated a clear tryptophan deficiency of the basal diet. Graded doses of tryptophan to the basal diet resulted in improvements in weight gain, daily feed intake and feed conversion rate. In pigs the differences in weight gain, feed intake and feed conversion rate between both tryptophan sources were not significant. In broilers feed intake with commercial L-tryptophan was higher than with Tryptosine. Weight gain, feed conversion rate and health state did not differ significantly between both sources. Protein deposition in broilers was significantly improved with dietary supplementation of 0.3 g/kg tryptophan, whereas the addition with 0.6 g/kg showed no further response. No differences were observed between both tryptophan sources. From the results of the two studies it was concluded that the biological activity of tryptophan from Tryptosine is equal to that of L-tryptophan for pigs and broilers.