Dendritic amine and guanidinium group-modified nanoparticles were investigated for the delivery of model peptide drug into primary osteoclast precursor cells (bone marrow macrophages; BMMs). The model peptide drug was encapsulated into the nanoparticle by dropping the drug/carrier dissolved in dimethylsulfoxide/methylene chloride cosolvent into water containing poly(vinyl alcohol) as a stabilizer. Flow cytometry and spectrofluorimetry analysis indicated that the model drug itself was not taken up by the BMMs; however, nanoparticle systems underwent significant cellular uptake. In particular, guanidinium group-modified nanoparticles were taken up more efficiently than amine group-modified ones. Cell viability studies showed that both amine and guanidinium group-modified nanoparticles exhibited no significant cytotoxicity up to 100 μg/mL against the cells.