A method is described for the rapid isolation of peribacteroid envelope-enclosed bacteroids (peribacteroid units) from soybean nodules. The isolated peribacteroid units were almost free from contamination by free bacteroids and nodule organelles, as judged by electron microscopy, and contained between 3 and 16 bacteroids per envelope (in cross section). Permeability of the envelope to various metabolites was screened by measuring their ability to stimulate endogenous bacteroid respiration. By this criterion the envelope is poorly permeable to oxoglutarate, glutamate, pyruvate and arabinose, all of which can be utilized by free bacteroids. Succinate, malate and fumarate are readily utilized by both peribacteroid units and bacteroids but the apparent Km for succinate and malate are higher in intact peribacteroid units. The results indicate that the peribacteroid envelope regulates the movement of metabolites into bacteroids.