A method is provided for enhancing transmembrane transport of a diagnostic agent across a membrane of a living cell. The method comprises contacting a membrane of a living cell with a complex formed between said diagnostic agent and ligands selected from biotin or biotin receptor-binding analogs of biotin, folate or folate receptor-binding analogs of folate, riboflavin or riboflavin receptor-binding analogs of riboflavin to initiate receptor mediated transmembrane transport of the ligand complex. The method is used for imaging tissues in vivo.
