Can you tell us what cells you are using.  Are you looking at channels or transporters?

I am looking for a general assay for chloride transport in which my positive control is DIDS (4,4'-diisothiocyano-2 ,2'-stilbenedisulfonate) - a known anion-transport inhibitor. I am working with HeLa cells, though I can check also in other cell lines.

Thank you.

I would look into the literature concenning CFTR the chloride channel that is mutated in cystic fibrosis

Unfortunately there really isn't a general chloride transport assay.  36Cl could be used but its not easy to get hold of.  You really have to figure out what kind of transport process you are looking at.  While DIDS certainly blocks some chloride transport, it inhibits lots of things, and other chloride transport processes are not affected by DIDS.  First, you have to thing about whether you are looking for a channel activity of a transport activity.