The interphotoreceptor space (IPS) of the retina is bordered by the retinal pigment epithelium, photoreceptors, and Müller cells and surrounds the photoreceptor outer and inner segments. It contains a matrix composed of glycosaminoglycans and proteins, including interphotoreceptor retinol-binding protein (IRBP). The matrix does not diffuse sclerad through the tight junctions that link cells of the pigment epithelium or vitread beyond the point at which photoreceptors and Müller cells are linked by zonulae adherentes that comprise the external limiting membrane (ELM). Biotinylated protein probes of known Stokes' radius were used to determine the pore size of the ELM. Following exposure of the photoreceptor side of isolated rabbit retinas to each protein, the extent of diffusion of the probe through the retina was determined by avidin D-horseradish peroxidase histochemistry. Each protein with a Stokes' radius of 30 A or less diffused freely through the neurosensory retina while each protein with a Stokes' radius greater than 36 A was blocked abruptly at the ELM. Thus, the pore radius of the zonulae adherentes of the ELM lies between 30 and 36 A, which is sufficiently small to account for containment of IRBP (55 A) within the IPS. This study emphasizes that in addition to providing structural support, the zonulae adherentes of the ELM serve to define an important extracellular space of the retina. This has clinical relevance, since two serum proteins tested, albumin and gamma-globulin, are too large to diffuse through an intact ELM. This may explain why protein-rich fluid accumulates in the IPS when the outer blood retinal barrier is compromised by disease or injury.(ABSTRACT TRUNCATED AT 250 WORDS)