After lapse of a few months, we recently attempted insulin-tolerance tests in C57BL/6N female mice at 6 months of age. We began, years ago, with Humulin-r at 0.75 U/kg. In our recent tests we used the same protocol, but the only response was a slight increase in blood glucose! We tried several higher doses. At 1.75 U/kg a very slight decrease was observed, but not until 60 minutes. We purchased a non-clinical preparation of insulin from a company that sells a lot of growth factors and cytokines. It produced the puzzling increase in blood glucose at both 0.75 and 1.25 U/kg. Has anyone else had a problem with insulin efficacy of late? I can't believe all our mice are suddenly insulin resistant!