Utilization of Leucaena leucocephala de Wit, a tropical origin perennial, as feed has been promoted, but toxicity of the mimosine in it, one of the free amino acids found in the plant, has been a serious problem in usage promotion. In a series of studies to get the basic information required for utilizing this plant as feed, the mimosine content of each segment was determined using a conventional method, and we have developed a new method to isolate and purify mimosine from its seeds through a procedure which is simpler than the conventional one. To identify the physiological activity of the mimosine thus prepared, its effects on body weight, feed intake and pelage growth of mice (Jcl-ICR) were investigated. The results obtained were: 1) The average mimosine content of the plant was about 3.6%, while segments with an active growth showed a higher value. 2) Sixty percent of the total free amino acids extracted was found to be mimosine. 3) The hot water extract of about 50g of the seeds was deproteinized, concentrated and repeatedly crystallized to obtain needle shaped mimosine. The yield was approximately 0.95 g. These crystals were identified to be pure mimosine on thin-layer chromatography, UV spectrum, IR spectrum and amino acid analysis, using a commercially available mimosine as a standard. 4) The mice were fed with rations containing 0.5%, 1.0% or 2.0% mimosine. With an increase in mimosine level, the animal body weight was reduced, and mimosine administration of more than 0.5% was considered to result in feed intake inhibition. 5) With increased mimosine concentration in rations, the pelage growth of the mice also considerably decreased. From these results, the isolated and purified mimosine was identified as the same substance as the commercially available mimosine on the basis of the chemical data and physiological activities observed.