Bradford reagent contains either Coommasie blue or brilliant blue. Hence the blue precipitate. Centrifuge the precipitate and take the reading. Don't sit breaking your head for the reason for the precipitate.

I respectfully disagree with Jai Ghosh. I believe the blue precipitate consists of protein (and possibly other substances) that precipitated because of the acidity of the Bradford reagent. As a result of this, the measurement of protein concentration will be. It would be a good idea to switch to a different protein assay method, such as BCA. An added advantage is that the detergent in your diluted sample will not cause interference with the BCA assay, as it does in the Bradford assay.

Dear. Jai Ghosh

Thank you for your kind concern. But those 'precipitation' that I said is something different from blue aggregation that normally formed after proteins loaded on bradford reagent, which can be easily dissociated with brief shaking. I prefer bradford assay for 96well plate format to measure the absorbance of a large numeber of protein samples in two duplicate wells. So those indissociable precipitates led to very different absorbance measurement from two identical duplicate wells,which I found difficulties in.

I wish you would understand my situation.

Still, thank you for your concern.

Have a nice day!

Dear Adam B Shapiro

Thank you for your adivice.

Have a nice day.

Very true what Adam says. then the best alternative would be to switch over to other methods like Lowery's method. You may start with absorbance measurement at 280 nm in a spectrophotometer.

The BCA assay can also be done in 96-well plates. The usual method involves an incubation at elevated temperature. You can do this by putting an adhesive foil sealer on the plate (to prevent evaporation) and putting it in a cell culture incubator at 37oC for 30 minutes, or you can leave it at room temperature for 2 hours.