I'm having problems with immunoprecipitation followed by westernblot experiments. If I immunoprecipitate with a mouse Ab and blot with a rabbit Ab the secondary anti-rabbit-HRP antibody picks up the mouse Ig on the membrane. Same problem if I IP with a rabbit Ab and blot with a mouse Ab - the anti-mouse-HRP picks up the rabbit Ig on the membrane. And wouldn't you know it, the protein I'm interested in is about 50 kDa. I am using my immunoprecipitating Abs at 1:5000 dilution (monoclonal culture supenatants or rabbit serum) and the secondary-HRP conjugated Abs at 1:10,000.I have tried secondary HRP conjugated Abs from various companies with no luck.