We present a rapid and specific procedure that enables one to identify living target cells of peptide hormones within heterogeneous cell populations by means of morphological demonstration of ligand-receptor binding. This is exemplified using a biotinylated parathyroid hormone (PTH) analogue (biotinyl-b-PTH 1-84) which reacts with avidin-fluorescein (avidin-FITC) as a histochemical marker. The experiments revealed a fine dot-like distribution pattern of binding after 1-5 min of incubation at room temperature, changing to a more clustered pattern after 10 min of incubation. Competition of labeled and unlabeled PTH exhibited lack of staining if unlabeled PTH was applied in excess. The results suggest that the demonstrated binding sites represent specific receptors for PTH on living target cells.