Biotinylated bleomycin A(5) was attached to streptavidin-derivatized microbubbles, and a solution containing the conjugate was passed over a monolayer of cultured MCF-7 cells. The bleomycin-derivatized microbubbles adhered to the MCF-7 cells, and the association could be monitored by the use of a microscope. Three other cancer cell lines gave similar results. The bleomycin-microbubble conjugate did not bind to a normal breast cell tine (MCF-10A) or to the matched noncancer cell lines corresponding to the other cancer cell lines targeted by bleomycin. No binding to any tested cell tine was observed when the microbubbles; tacked conjugated bleomycin A(5) or when the microbubble contained a bleomycin A(5) analogue tacking the carbohydrate moiety.