An automated fluorometric method, including norharmane transformation of tryptophan (TRP) by formaldehyde and perhydrol, is applied to free and total TRP determination in human plasma. Separation of free TRP determination in human plasma. Separation of free TRP is achieved by ultrafiltration at pH 7,25 under an air/CO2 flow. The procedure here described is easy, rapid and very sensitive, allowing to work on microsamples if free TRP separation could be realized with them. A correct precision is obtained (CV : 2,8% for plasma repetability) and the specificity is quite good in plasma according to the negligible effect of tryptamine. Frequent values, calculated from 19 healthy subjects are [mean +/- tSE (risk 5%)]:--free TRP : 11,8 +/- 1,5 mumol/l;--total TRP : 52,2 +/- 19,7 mumol/l.