casein as blocking agent - which one to use? (Sep/25/2007 )

Hi,
i'm trying to optimize and immunoassay, and as the sample is going to be human serum have just started looking into alternative blocking agents (was using BSA before, but read in this forums not a good idea when sample is serum).
my question now is, when looking at the sigma website for casein, 84 different products come up. can someone tell me which is the one i need?

thanks in advance!!!!!

i'm not sure which is best but you will have problems solubilizing the technical grade. we had to acidify to solubilize.

sigma does have a product called "casein blocking buffer". they say it is good for use with nitrocellulose and nylon membranes.