Lecithins from livers of normal rats and rats deficient in essential fatty acids were separated into fractions containing mainly 1, 2, 3 or 4 double bonds. The fractions thus produced were dispersed in aqueous buffer by ultrasonic irradiation and their rates of binding to mitochondria, whose lipids had been extracted with aqueous acetone, were measured. The kinetics of this reaction can be explained by an initial Brownian coagulation between lecithin and protein particles followed by a rate limiting interaction of lecithin molecules with the mitochondrial protein. A rate equation for these two steps was derived containing three parameters which represent respectively the rate constant for the coagulation step, the surface area of the mitochondria and the rate constant of the second step. The experimental data was fitted to this equation. The effect of variation in fatty acid composition was most marked on the value of the rate constant of the second step, the interaction of lecithin with mitochondrial protein. Lecithins containing arachidonic acid interacted significantly more slowly than those containing 5, 8, 1 l-eicosatrienoic acid or mixtures of oleic and linoleic acids. The significance of this difference is discussed in relation to the structural function of the essential fatty acids.