The 5′ upstream region of rat PAI-1 promoter is required for induction in a heterotypic coculture. A series of 5′ deletions of the 2.4-kb rat PAI-1 promoter31 were cloned upstream of a CAT reporter gene and transfected into L-cells. For each construct several transfected clones were selected and grown either alone or after mixing with LE-II endothelial cells (in a 1:1 ratio). Cultures were grown for 5 days (ie, at least 1 day after reaching confluence) and then obtained and analyzed for CAT activity. A comparison was made between the heterotypic cocultures (Ht) and an equal number of cells of the respective L-cell clone to which LE-II cells were added (both obtained from individually grown cultures) (Ho). Results are expressed as an Ht/Ho ratio and are the average of six different clones for each construct.