Polychlorinated biphenyls (PCBs) are highly lipophilic molecules that are now widely distributed throughout the environment. They tend to accumulate in the adipose tissue and thus remain as a source of continuing exposure to the rest of the body. The interaction of some of the representative PCBs with phospholipid bilayer membranes was monitored by the spin label technique. Under identical experimental conditions, it was found that an increase in chlorine substitution in the 2,2’, 6,6'‐positions of the biphenyl rings enhances the motion of the spin label in the bilayer membrane. It is proposed that the perturbation which increases the membrane fluidity can be correlated to the degree of non‐coplanarity of the biphenyl rings incorporated into the bilayer membranes.