Define "best".  FACS gives you data about individual cells - you put an antibody for each target of interest into your cells, feed it into the instrument, and then play with the output to make it look how you want, essentially.  Each cell is measured individually for each antibody, so you get a whole lot of data.  But it's comparatively expensive, as techniques go, at least if you don't have your own instrument and need to use a facility.

ELISA by comparison is pretty cheap, and can be read out on non-specialized plate readers, but tends to be a bit harder to optimize.  Once you do optimize, though, you have a fairly robust, fairly quantitative assay available.  But it only gives you bulk numbers - it tells you information about your cell population as a whole, not about individual members.  It's probably a bit slower than FACS too, unless you have robots to do everything for you and can just come back the next day for data.

Which is better really depends upon what sort of data you need, and how much you want to spend to get it.

ELISA. !

Thanks for reply. I will think about it.

When there is a cytotoxic lymphocytic activation you can find an increase of CD3-CD8+ lymphocytes. In cancer patients there may be an increase in T CD4 + lymphocytes and may be a sign of an effective cytotoxic immunity (TH17).

If you have access to a flow cytometer, it would be better for observing many parameters all at once...including the health (viability) of the cells.

I agree with previous recommendations. Levels of INF, TNF, granzyme and Perforin can be easly quantitatively determined by comercial ELISA kits. FACS of course is of help but it deatermine amount or number of cytotoxic cells but NOT THEIR POTENTIAL.

Flowcytometry is better, as you can determine multiple parameters at a time even from a single cell as suggested in the previous post. As far as quantitation is concerned one can look for MFI data in flowcytometry. That will give you an indication about their cytotoxic potential as well.

Thanks for helpful comments.