In the present study, we evaluated the effects of a neutralizing antivascular endothelial growth factor (anti-VEGF) antibody on angiogenesis and growth of tumor spheroids using an intravital microscopic technique permitting noninvasive, in vivo and in situ study of tumor angiogenesis and tumor growth in conscious mice. Tumor spheroids of the human rhabdomyosarcoma cell line A673, with a diameter between 600 and 1000 microns, were implanted in dorsal skinfold chambers inserted on Beige nude/xid mice. Tumor cells were prelabeled with a fluorescent vital dye [(5-(and-6)-((4-chloromethyl)benzoyl)amino)tetramethylrhodamine], which allowed estimation of the growth of the implanted tumor spheroids. Treatment (i.p.) with the monoclonal antibody A4.6.1, specific for VEGF, completely inhibited neovascularization of the microtumors and suppressed their growth to the extent that tumors implanted in treated animals leveled off at a volume less than 1 mm3, i.e., the anti-VEGF antibody dramatically changed the growth characteristics of the tumor line from being a rapidly growing malignancy to a dormant microcolony.