In this study, selective cancer cell targeting of biodegradable poly(lactic acid) (PLA) nanoparticles (NPs) has been investigated in vitro. SKOV-3 (HER2 positive) ovarian cancer and Daudi (CD20 positive) lymphoma cell targeting was mediated by anti-HER2 (trastuzumab, Herceptin) and anti-CD20 (rituximab, Mabthera) monoclonal antibodies (mAbs), respectively. The mAb against nonexpressed antigen serving on each cell as isotype matched irrelevant control. Two different targeting approaches have been studied, a direct method using antibody-labeled NPs (mAb-NPs) and a pretargeting method using the avidin-biotin technology. For the direct protocol, fluorescent PLA-NPs were prepared including 10% 1-pyrenebutanol (PB)-labeled PLA in the NP-preparation (PB-NP). Thiol groups were covalently bound to the PB-NP, and the resulting thiolated PB-NP were coupled with the two mAbs using a bifunctional cross-linker. The effective targeting of cells by mAb-PB-NP was shown by flow cytometry analysis. Clearly anti-HER2-PB-NP specifically bound to the SKOV-3 cells and not to the Daudi cells, while anti-CD20-PB-NPs bound to Daudi cells but not to SKOV-3 cells. Specific mAb-PB-NP binding to tumor cells produced a mean 10-fold or higher signal increase compared to irrelevant IgG-PB-NPs. For the pretargeting protocol, plain PLA-NPs were also thiolated and NeutrAvidin-Rhodamine Red-X (NAR) coupled to the functionalized PLA-NPs with sulfo-MBS. The two-step method was evaluated in vitro by incubating SKOV-3 cells first with biotinylated mAbs followed by NAR-NPs. The relative fluorescence associated to the specific binding of NPs produced a 6-fold increase in flow cytometry signal compared to nonspecific binding. In conclusion, these experiments have shown that NPs covalently coupled with antibodies or NAR can specifically and efficiently bind to cancer cells in both a pretargeting and a direct approach, suggesting that functionalized NPs may be a useful drug carrier for tumor targeting.