Here is a response from our Tech Support team:

Regarding the usage of 12-9957-42, we stain the U937 cell line, both unstimulated and stimulated. We stimulate cells overnight with freshly made 50ng/ml GM-CSF and 100ng/ml hIFNg.

The staining protocol is our standard surface staining protocol for Staining Cell Surface Antigens, PDF: http://www.ebioscience.com/media/pdf/best-protocols/staining-cell-surface-antigens-for-flow-cytometry.pdf

Thanks,
Wendy