In order to detect intracellular antigens, cells must first be permeabilized especially after fixation with cross-linking agents such as formaldehyde and glutaraldehyde. Permeabilization provides access to intracellular or intraorganellar antigens. Two general types of reagents are commonly used: organic solvents, such as methanol and acetone, and detergents such as saponin, Triton X-100 and Tween-20. The organic solvents dissolve lipids from cell membranes making them permeable to antibodies. Because the organic solvents also coagulate proteins, they can be used to fix and permeabilize cells at the same time. Saponin interacts with membrane cholesterol, selectively removing it and leaving holes in the membrane. The disadvantage of detergents such as Triton X-100 and Tween-20 is that they are non-selective in nature and may extract proteins along with the lipids. This chapter provides methods for the use of organic solvents and detergents to permeabilize cell membranes.