Lectin binding patterns to the developing pulmonary vasculature were studied in 10 Large White pigs aged 1 min to 1 week and in three adult animals. Paraffin-embedded tissue sections were exposed to eight lectin peroxidase conjugates: Dolichos biflorus, Triticum vulgaris, Concanavalin A, Ricinus communis type 2, Arachis hypogaea, Ricinus communis type 1, Tetragonolobus purpureas and Ulex europeus. Lectin binding patterns to the pulmonary arterial and venous endothelium, to smooth muscle cells (SMCs) and to the arterial connective tissue were age-related. Changes occurred during the first week of life and between 1 week and adult life. Neither the endothelial binding patterns in the adult nor the SMC patterns in the immature and adult lung conformed to known morphological differences between the different segments of the arterial and venous pathways. Heterogeneity for endothelial binding was seen in the immature lung. These studies indicate biochemical differences in surface structure between the endothelium, SMCs and connective tissue of the immature and mature lung. Ultrastructural localization of the lectins in the vasculature of the developing animal lung ought to help interpret similar data obtained on the vessels of the immature human pulmonary hypertensive lung using lectins which show similar binding patterns in both species.