Abstract: For the determination of total hydroxyproline, 100 µL of plasma or 1 mL of urine was mixed with 200 µL of 12 M HCl and autoclaved for 3 h at 120°C. The hydrolysate was neutralized with 1 mL of 12 M KOH and 1 mL of 1 M L-cysteine. A 100 µL portion was injected into a carrier stream in a FIA system of chloramine T in borate buffer and KCl adjusted to pH 8.7 with 1 M KOH. The mixture was heated at 120°C in a mixing coil (24 m x 1 mm i.d.) in an Al block. The reaction mixture was merged with a stream of Ehrlich's reagent (1:1), the resulting solution passed through a reaction coil (10 m x 1 mm i.d.) to a double beam photometer for detection at 560 nm. Free hydroxyproline was determined as above but omitting the hydrolysis step and filtering the serum. Calibration graphs were linear up to 1.22 mM with a detection limit of 3.8 µM. The within-run RSD was 2.34, 2.25 and 2.53% for 76, 38 and 19 µM, respectively. The recovery of 10^-50 µM of hydroxyproline in urine was 92-104%. Results agreed well with those obtained by HPLC.