The tissue concentration of an endogenous beta-galactoside-specific lectin is developmentally regulated in rabbit tissues during foetal and neonatal development. Immunoassay data and localization studies both indicate that the lectin is particularly associated with late embryonic and early postnatal development. No lectin could be detected in embryos prior to 21 days of gestation, whereas many tissues showed an increase in lectin around the time of birth. It is likely that in most tissues the high concentrations of lectin are largely due to increased synthesis by fibroblasts, which are particularly abundant and active in connective tissue during the extensive tissue reorganization taking place at this time. The lectin appears to be synthesized by other differentiated cell types also, notably myoblasts, alveolar cells and erythroblasts. The peak of lectin concentration seen in foetal liver probably reflects lectin associated with foetal erythropoiesis. The rabbit lectin has a low specific activity but its tissue concentration is correspondingly higher than lectin concentrations in other mammals. This conservation of total lectin activity suggests a fundamental role for the lectin dependent upon its saccharide binding activity. This and other indirect evidence suggests that these lectins are involved in the synthesis, secretion or organization of extracellular matrix components.