Griffonia simplicifolia lectin (GSL I+II) is fractionated from a woody climbing shrub native to Africa. It consists of both lectins GSL I+II. GSL I+II shows specificity for blood group B greater than blood group A1. This lectin interacts with pulmonary microvascular endothelial cells, but does not readily interact with pulmonary arterial endothelial cells. This binding pattern has been successfully utilized to enrich cell populations in vitro that possess functional behaviors characteristic of those seen in vivo.