Doxorubicin (DOX), a common cancer chemotherapeutics, was conjugated to folate-modified thiolated-polyethylene glycol-functionalized gold nanoparticles. The in vitro, controlled release behavior of DOX-loaded gold nanoparticles was observed using porous dialysis membranes (cut-off = 2 kDa). DOX-loaded gold nanoparticles had higher cytotoxicity for folate-receptor-positive cells (KB cells) compared to folate-receptor-negative cells (A549 cells) which were 48 and 62% viable for 10 mu M doxorubicin, respectively. This indicates the potential of these nano-carriers for targeted-delivery. In addition, healthy cell viability was 69% for 10 mu M free doxorubicin whereas for the same content of drug in DOX-loaded nanoparticles healthy cell viability increased to 80%.