1. Add an appropriate amount of casein to the buffer in a glass beaker with a magnetic spin bar. (Recommended concentration is 0.5%; maximum soluble casein is 1%.)
2. Place the beaker on a magnetic stirrer/hot plate.
3. Stir thoroughly while heating until the solution becomes translucent. Avoid foaming.
4. Remove the beaker from heat and let the solution cool. In some cases, it may also be desirable to filter the solution with a 0.45 to 1.2 µm pore size filter. For prolonged storage of casein solutions, refrigeration and preservatives (Thimerosal^TM) are recommended.

1. Place the membrane to be blocked in an appropriately sized container.
2. Add a sufficient volume of casein solution to cover the membrane (0.1 to 1.0 mL/cm² of membrane).
3. Place the container on top of a shaker table and agitate. Avoid foaming. 
4. Allow the membrane to incubate in the blocking solution for 15 to 60 minutes. Note optional rinsing step:
1. Drain off blocking solution. 
2. Add buffer (PBS or other) to dish (0.5 to 2.0 mL/cm² of membrane). 
3. Agitate for 5 minutes. 
4. Drain off buffer.
5. Blocked membrane may either be dried at room temperature for one hour in a forced air oven (less than 37 °C) for 15 minutes. Addition of surfactant 0.1% Triton* X-100 or 0.05% Tween-20* is recommended if the membrane is to be dried after blocking. The surfactant will assist blocking efficiency and rehydration of the membrane.

*Triton is a registered trademark of Rohm & Haas, Philadelphia, PA, USA. Tween-20 is a registered trademark of ICI.