A pattern photobleaching method has been used to measure the rates of lateral diffusion of fluorescent-labeled specific anti-nitroxide IgG bound with both combining sites to nitroxide-containing phospholipids in liposomal membranes composed of dimyristoyl phosphatidylcholine at 28 degrees C ("fluid"), dipalmitoyl phosphatidylcholine at 32 degrees C ("Solid"), and dipalmitoyl phosphatidylcholine containing 15 or 25 mol% cholesterol ("solid" or "fluid," respectively, at 32 degrees C). The diffusion coefficients of the bound immunoglobulin were found to be the same as those of fluorescent-labeled phospholipids in each case even though these diffusion coefficients range from 10(-11) to 10(-8) cm2/sec. Hapten-containing liposomal membranes of the type studied here have previously been shown to elicit a number of antibody-dependent immune responses. Therefore, this work indicates that membrane-bound but otherwise freely diffusing antibodies are sufficient for these reponses.