Although 14C-labelling has been routinely used for small molecules, this technique is not routinely applied to therapeutic proteins due to difficulties of incorporating the label into the protein to a sufficiently high specific activity. An analytical method known as accelerator mass spectrometry (AMS) offers an extremely sensitive method of 14C quantification, thereby enabling (14)C-labeling methods to be applied to therapeutic protein detection. The therapeutic protein CAT-192 (metelimumab), a human anti-TGFss1 monocloncal antibody was manufactured in the presence of 14C-precursors resulting in a low specific activity product (1.4% 14C incorporation). [14C]-CAT-192 was administered to rats (1mg/kg and 222, 22 and 2.2 dpm/kg) and serum samples were collected. 14C in serum samples from the 2.2 dpm dosing was not detectable but samples from the 22 and 2220 dpm doses were measured by AMS and by ELISA for comparison. By both ELISA and AMS bioassay, the half-lives approximated 140 h (S.E.M. 15 h). The estimates of clearance were also comparable, 7.3 and 4.6 x 10(-4)ml/h/g (S.E.M. 6.6 and 5.1 x 10(-5)) for ELISA and AMS, respectively. The estimated limit of quantification (LOQ) was approximately 1 ng/ml, about 15 times lower than the ELISA LOQ of 15.6 ng/ml.