Therapeutic use of hydroxycamptothecin (HCPT), a promising antitumor agent, is limited by its poor solubility and rapid destruction. Amphiphilic block copolymer micelle carriers possess significant potential for improving drug solubility and stability. Poly[ethylene glycol]-poly[gamma-benzyl-L-glutamate] (PEG-PBLG) micelles were prepared and loaded with the active lactone form of HCPT using an uncomplicated dialysis method. HPLC and scanning electron microscopy studies revealed an encapsulation efficiency of 56.8% and a core-shell figure with a mean diameter of 200 nm. Encapsulated HCPT lactone was compared with the less active, open ring-carboxylated HCPT-Na+ soluble form generated in vivo from the free active lactone for activity against oral squamous cell carcinoma. Cytotoxicity in vitro was measured in cultured Tca8113 cells by the MTT assay and microscopy techniques. The golden hamster cheek pouch squamous cell carcinoma model was employed for in vivo studies; encapsulated lactone and open ring-carboxylated forms of HCPT were administered intraperitoneally, followed by determinations of tumor growth rate and inhibition ratio. PEG-PBLG micelles were not cytotoxic in vitro. At 48 h of treatment, open ring-carboxylated HCPT proved significantly more cytotoxic in vitro than encapsulated HCPT lactone. At 96 h, however, the open ring-carboxylated and encapsulated drugs displayed comparable in vitro cytotoxicities. In the in vivo squamous cell carcinoma model, encapsulated HCPT lactone produced greater and more prolonged tumor suppression compared to the open ring-carboxylated form. The antitumor effects of HCPT/PEG-PBLG micelles against oral squamous cell carcinoma in vivo are concluded to be superior to those exerted by open ring-carboxylated HCPT.