Before determining the IC50’s for ten potential PTP1b inhibitors, we performed some initial experiments to establish appropriate assay conditions. We assessed conditions for a full length human PTP1b and a truncated human PTP1b (Glu2-Asn321). First we titrated the PTP1b’s using 5 mM pNPP as the substrate. From these titrations we decided to use 50 ng/reaction for the full length PTP1b and 40 ng/rxn for the truncated PTP1b. We next titrated DMSO to determine the highest concentration that would be tolerated by each PTP1b. The Km of pNPP for each PTP1b was then measured in the presence of the highest tolerable DMSO concentration. Finally, the IC50 for a known tyrosine phosphatase inhibitor, Na3VO4 was measured for each PTP1b. For the full length PTP1b, we determined the maximum DMSO concentration to be 15 v% and the Km to be 0.7 ± 0.04 mM. For the truncated PTP1b, we determined the maximum DMSO concentration to be 7.5 v% and the Km to be 1.3 ± 0.1 mM. Using 0.7 mM pNPP, we observed an IC50 = 19.3 ± 1.1 µM for Na3VO4 for the full length PTP1b. Using 1 mM pNPP, we observed an IC50 = 54.5 ± 1.1 µM for Na3VO4 for the truncated PTP1b.