By use of double-labelling indirect immunofluorescence, we have shown that the major mammalian testicular glycolipid sulfogalactosylacylalkylglycerol (SGG) and a membrane protein, previously shown to bind specifically to SGG in vitro, are colocalized on the surface of rat testicular germ cells during spermatogenesis. SGG is restricted to convoluted membrane domains within these cells. Thus, the binding affinity in vitro is reflected in the cell surface topology. The topological relationship between these two antigens was also studied during epididymal sperm maturation. Whereas these antigens were colocalized in caput spermatozoa (on the middle and principal piece of the tail and on the concave surface of the head), the distribution of the binding protein was altered for cauda sperm in that the convex surface of the sperm head was now strongly labelled. These studies illustrate the dynamic nature of protein-glycolipid interactions during germ cell differentiation.