This report details experimental results which show the presence of enzymic aminopeptidase-N-like activity on endothelial cells, concomitant with cell surface expression previously detected by both ELISA and indirect immunofluorescence. This activity, detected using selected chromogenic substrates in 'functional' assays, is shown to be due (at least in part) to a molecule previously termed 'gp 150' and recognized by MoAb belonging to CD-13, since such antibodies can be shown to inhibit this activity. Activity, as detected on endothelial cells, is similar to that observed on various haemopoietic cells. These assays not only provide a functional basis to cell surface gp150 (CD-13) co-expression on haemopoietic and endothelial cells but have also been used to help define structural epitopes present on aminopeptidase-N/gp150, previously analysed using radiolabelled antibodies in competitive binding assays. Appraisal of these new data suggests that the number of distinct antibody binding sites on this molecule is greater than that previously demonstrated. This study is therefore an important first step in investigating the potential involvement of this selective peptidase molecule in the control of haemopoietic cell growth and differentiation and in haemostatic mechanisms.