Purified lipids (ovolecithin, cholesterol, dicetylphosphate, in 70:10:20 molar ratios) swell spontaneously in salt solutions to form spherules which behave as saltcontaining compartments limited by discrete membranes. These structures exhibit permeability characteristics for cations, anions, and water which are qualitatively similar to those occurring across natural membranes. After spherules were permitted to form in the presence of 0.145 M H 2P0 4 −, CrO 4 2−, glucose, or glycine, any excess marker ions or molecules which remained uncaptured in the aqueous phase between lamellae were removed by dialysis against 0.145 M NaCl/KCl. Subsequently, spherules were exposed to steroids (5 mM), Triton X-100, and streptolysin S, agents that affect the membranes of lysosomes, mitochondria, and erythrocytes. Progesterone, desoxycorticosterone (DOC), diethylstilbestrol, and 5 β-H steroids such as etiocholanolone, considerably accelerated the leakage of anions, glucose, and glycine into the surrounding medium, as did Triton and streptolysin. In contrast, cortisone, cortisol, and chloroquine, which protect natural membrane-bounded structures such as lysosomes, retarded leakage of the markers. It was possible to form spherules that contained cortisone preincorporated into the lipid lamellae. At an optimum molar preincorporation of 1 per cent, structures that contained cortisone (but not its inert metabolite tetrahydrocortisone) proved more resistant to leakage induced by other steroids. Negatively stained preparations in the electron microscope showed that the discrete spherules were formed of multiple concentric lamellae which were separated by compartments of approximately 50 Å distance. Triton X-100 completely disrupted these structures: only amorphous debris was formed. In contrast, DOC-treated samples showed the emergence of new forms: although small, discrete spherules remained, the predominant forms were a series of elongated, tubular, strand-like structures, loops and whirls of which maintained a lamellar substructure. These studies suggest that steroids interact with artificial lipid spherules to produce changes in their permeability which resemble steroidinduced changes in natural membranes, most likely by provoking a structural rearrangement of lipid layers.