Determination of residual BL21 proteins upon affinity purification by Western Blot. A protein sample of a affinity purified 40kD His-tag protein (A) expressed in the E.coli expression strain BL21 as well as a BL21 whole cell lysate (B) were resolved by SDS-PAGE and subsequently transferred to PVDF membrane. The target protein was determined by antiHIS-tag in combination with Anti-E.coli (BL21) antibody followed by Anti-Rabbit-HRP and colorimetric detection. The arrows indicate residual BL21 proteins in the purified fraction. Lane B shows the reactivity of Anti-E.coli (BL21) against a BL21 protein lysate. The numbers shown on the right represent marks of the protein standard used in the experiment.