SummaryThreonine has been determinated quantitatively by directly titrating against nickel sulfate separately and in combination with L-arginine and together with L-histidine and L-arginine without separating in one solution. Maximum error for threonine is 1.2 %. The reaction between nickel sulfate and threonine resulted in the formation of a complex in the ratio of 1.2. In combination with L-arginine threonine is first titrated; whereas in the form of a mixture with L-histidine and L-arginine the order of titration starts from L-histidine, threonine and then L-arginine.