Serotonin synthesis critically depends on plasma levels of tryptophan (TRP). Earlier studies have shown that for mood and cognitive benefits to occur, the ratio between TRP and other large neutral amino acids (LNAA) has to be increased by approximately 40 %. The present study investigated the dose-dependent effects of a TRP-rich hydrolysed protein (egg-protein hydrolysate, EPH) on the plasma TRP:LNAA. Moreover, it was investigated whether EPH could increase TRP:LNAA in the presence of 2 g of milk protein (MP). In a randomised double-blind crossover design, plasma amino acids were measured every 30 min for 3·5 h after ingestion of a drink containing either three different doses of 4, 8 and 12 g EPH containing 270, 560 or 800 mg of TRP, respectively, the combination of 4 g EPH and 2 g MP (74 mg TRP), or 4 g MP (148 mg TRP) in twenty healthy subjects with a mean age of 52 years. All three EPH doses caused significant increases of TRP:LNAA above 40 % at 30, 60 and 90 min after consumption in a dose-dependent manner. Compared with the 4 g EPH, the increase in TRP:LNAA in the 4 g EPH with 2 g MP condition was significantly lower at 60 min (63 v. 44 %, P < 0·001) and did not differ significantly at 90 min (58 v. 53 %, P>0·05). The present study showed that a low dose of 4 g EPH with even the addition of 2 g MP was sufficient to increase the ratio of TRP:LNAA above 40 %. Thus, EPH offers a viable ingredient to increase TRP availability.