Reports on a high-precision comparison at the 0.2% to 0.3% level conducted with samples supplied by the radiocarbon laboratory of the Quaternary Research Center at the University of Washington (QRC). Some four samples with ages ranging from modern to greater than 50,000 BP were dated in a blind test. The absence of cosmic-radiation background in accelerator mass spectrometry (AMS) dating is a major advantage for dating samples greater than 35,000 BP. The reliability of AMS dates greater than 35,000 BP depends entirely on understanding the contamination processes. By comparing results with those from laboratories capable of sample enrichment, such as QRC, it is possible to identify and estimate the intrinsic carbon-14 in the background samples as well as the contamination introduced by sample preparation.