signal transduction - inconsistency on growth factor signaling (Jan/02/2005 )

I'm studying intracellular signal transduction pathway using VEGF as ligand.
My results using the same conc. of ligands (from Invitrogen and Sigma) for the same period of time yielded different results on ROS production, MAPK activation and reporter gene activation????

Which one should I trust - Invitrogen or Sigma?
Postdoc2130, what is your experience on this? Thanks.

Previously when I worked for a biotech company, we have done a series of evaluation on various growth factors/cytokines from different suppliers like Invitrogen, Sigma, Calbiochem, Peprotech, R&D Systems, Epoch Biolabs....to search for a reliable, stable and consistent source for our HTS (high throughput screening) cell-based assay. What we found is that R&D Systems and Epoch Biolabs are among the best vendors in terms of product quality, specificity, activity and stability. We chose Epoch Biolabs finally because the cost-effective issue (theirs are cheaper).

That's my experience, hope it helps. Another way is that you can always compare your data with the published results if that available.