CellMask causing cell rounding

My lab and another lab have observed that staining MDCK and MEF cells with CellMask membrane stain (Deep Red version, if it makes any difference) causes the cells to round and, in some cases, eventually detach. I was wondering if anyone else has noticed something similar.

We’re using the 0.5x - 1.5x concentrations recommended by the product sheet and 5 min incubations at 37degC. CellMask isn’t killing the cells because the cells will reattach and spread if introduced to another substrate. We haven’t tested any other cell types. It doesn’t matter whether the cells are attached to a cell culture dish or collagen-coated or Fn-coated glass.

Do you have to be deep red? Maybe your question should be about alternatives, as significant morphology changes are rather too worrying to keep using this dye. I have had good success with Rhodamine B Hexyl ester, aka Hexyl-Rhodamine. It is sold as a mitochondrial stain but lights up all membranes in the Rhodamine color range. It only fluoresces in membranes, which it freely crosses, resulting in good signal and no photobleaching. Disclaimer:I run a microscopy core facility and am not a cell culture expert.