By use of the cationic dye Cuprolinic Blue in a critical electrolyte concentration method, the lungs of mice ranging from the late fetal stage (17 days of gestation) to the puberal stage (27 days) were surveyed for their proteoglycans. A large Cuprolinic Blue-positive filament is present within the connective tissue of lungs of late fetal and young postnatal mice. It is mostly located at the boundary between large extracellular matrix structures and electron microscopically empty areas, but sometimes also at the surface of fibroblast-like cells. The stainability of the filament disappears after treatment with chondroitinase ABC or chondroitinase AC, but not after treatment with nitrous acid. The Cuprolinic Blue-positive structure appears to be most abundant around 2 days postnatally. From day 10 on, its number decreases dramatically, and it can be no longer observed in the lungs of 27-day-old mice.