Help!! BCA kit v.s Bio-Rad kit for protein assay - (Aug/12/2010 )

Dear all, I got some problems when I analyzed my protein samples.

There's one bacterial strain which would release some maybe protein products out of cells,

therefore, I would like to monitoring the protein concentration with different cultivating period.

At the beginning, I used the BCA protein assay kit to measure the protein concnetration,

every samples were filtered by a 0.22 um sterile filter, so there's no any suspended solid in the samples.

The absoprtion of BCA reagents mixed with samples increased when the strain was cultivated longer and longer.

Consequently, I made a guess that there must be lots of protein products in the bulk solution.

At the same time, I also used the Bio-Rad protein assay kit to double confirm the protein concentration,

but the analytical results by the Bio-Rad kit determination showed there's no any protein products in the supernatant.

The absorption of Bio-Rad mixture was similar to that of the standard which is without any BSA addition.

It's such a ridiculous result for me....Could anyone tell me what's wrong with my experiments?

Lots Thanks!!!!

Bio-rad kit has gone off? IIRC they are a coomassie blue dye based ones and will go off quite rapidly once opened.