The important biogenic amine, serotonin (5HT), was determined in whole blood, platelet-rich plasma (PRP), platelet-poor plasma (PPP), and plasma ultrafiltrate after simple deproteinization. Following ion-pair chromatography on standard or narrow-bore reverse-phase columns, 5HT and the internal standard
(N-methylserotonin-NMS) were detected by flourometry with absolute detection limits of 2–4 pg. Levels obtained for whole blood and PRP were in agreement with previous methods. However, mean (±SD) values obtained for platelet-poor plasma (PPP) of 578±277 pg/ml (N=7) were approximately 3-fold lower than the lowest previous reports. We also report the first determination of 5HT in plasma ultrafiltrate, having observed mean levels of 387±222 pg/ml (N=7).