Abstract: Although sodium fluoride（NaF）is a good anticoagulant for plasma glucose measurement,it has not been known yet whether NaF could be used as an anticoagulant in RIA for plasma insulin.We observed the effect of NaF on RIA for plasma insulin by the use of liquid-phase and solid-phase RIA,and cotnpared NaF with heparin as anticoagulant.It was found that in liquid-phase RIA the insulin level of plasrna with NaF de-creased by 38.7%vs that of serum;however in the solid-phase RIA the insulin level of plasrna with NaF var-ied with insulin concentration in samples:the plasma insulin level increased by 31.7%in the satnples of low insulin concentration（≤20 mU/L）and decreased by 10.6% in the high insulin concentration samples（＞20 mU/L）vs those in the rnatched serum samples.There was no difference in insulin levels between plasma spec-imen with heparin and serurn sample either by liquid-phase or solid-phase RIA. It was evident that NaF could interfere with RIA for plasrna insulin,although its mechanism has not been clear it will not be fit for an anti-coagulant in plasma insulin RIA.