Hi again, As Jim & Rafael both noticed, I did indeed have a typo in my first e-mail. Having corrected it, though, I still appear to see the same error -- which is probably a result of the bug Rafael noted would be fixed in the next version of affy. Meanwhile, the suggestion to do: > colnames(exprs(Data)) <- samples.names; works perfectly. Many thanks for the help! Paul ---------------------------------------- This mail sent through www.mywaterloo.ca