Concentration-time profiles of(125)I-labeled recombinant human interleukin-3 ((125)1rhIL-3) were determined by reverse phase high performance liquid chromatography (RHPLC) after intravenous and subcutaneous administration of the drug in 16 rhesus monkeys. The initial and terminalT (1/2) in plasma after intravenous of 30 mug/kg were (0.15 +/- 0.13) and (2.21 +/- 0.59) h, respectively. Terminal half-lives after 30, 90 and 180 mug/kg subcutaneous (s. c.) injections were 2. 0-3. 8 h. Area under concentration-time curves (AUC) following s. c. were roughly increased with dose, while CL(s) were similar among different dosages. The absorption rates were dependent on concentration at injected site. Bioavailability was about 0.7 after s. c. Rapid biodegradation was found in plasma. Distribution profiles of total radioactivity were as follows: the highest level was found in urinary system; levels in bile-enteric system, lymph nodes, bone marrow and spleen were near to that in plasma, and level in brain was the lowest. The RH-PLC analysis revealed that kidney was one of the major organs for biodegradation.