Recombinantly expressed VP1-virus-like particles (VP1-VLP) of human polyomavirus JC virus (JCV) were described recently as a new DNA transporter system. It was shown that DNA molecules could be packaged into VP1-VLP during a controlled chemical reassociation/dissociation process. In the present study VP1-VLP were studied as carriers for pharmaceutical substances. Propidium iodide (PI) was packaged into VP1-VLP as a reporter molecule. The PI-containing VP1-VLP could be detected directly by flow cytometry. The fluorescence intensity of the VP1-VLP depended strongly on the initial PI concentration. This packaging method is easy to handle and applicable to viruses and VP1-VLP which can be dissociated and reassociated chemically.