Tumor-directed drug delivery is a promising strategy in cancer treatment, and in this field, monoclonal antibodies constitute an important class of targeting vehicles. A critical issue in the design of targeting conjugates is the timing of the release of the cytotoxic payload, with the ideal situation being the release at the maximum tumor uptake of the targeting molecule. A site-specific radiolabeling technique was used to elucidate the biodistribution and in vivo drug release pattern of an antibody conjugate of paclitaxel (PTX, 1, Figure 1) in which the drug and the antibody moieties were connected by a succinate (SX) linker. In this new method, a metabolite of PTX, 3′-(4-hydroxyphenyl)paclitaxel (3′-OH-PTX, 2, Figure 1) was used as a tyrosine mimic for the synthesis of the drug site-labeled conjugate (DSL, [125I]-3′-OH-PTXSXC225). This was achieved by iodogen 125I-labeling of 3′-OH-PTXSX and subsequent conjugation to C225. The antibody site-labeled conjugate (ASL, PTXSX-[125I]-C225) was prepared by direct radioiodination of PTXSXC225. Biodistribution of these compounds was studied in Balb/c nude mice bearing DU-145 human prostate carcinoma xenografts. While the 4 and 24 h tumor uptake (in percent injected dose per gram of tissue, %ID/g) for [125I]-3′-OH-PTXSXC225 were 3.3 ± 1.5 and 1.7 ± 0.6%ID/g, the PTXSX-[125I]-C225 showed tumor uptake values of 3.8 ± 4.2 and 14.8 ± 4.2%ID/g at these time points. This difference in the tumor uptake over time indicates an early cleavage of the drug with respect to the antibody tumor localization. This was further confirmed by an in vitro drug release kinetics study leading to a half-life of about 2 h for PTXSXC225 under physiological conditions. To increase the stability of the PTX-MAb bond, a new conjugate (PTXGLC225) with glutaric acid (GL) as the linker was synthesized. Under the same conditions, the PTXGLC225 showed a 16-fold increase in the half-life (t1/2) of the drug release. The effect of the increased t1/2 of this compound on the antitumor activity of the conjugate was tested in a DU-145 human prostate tumor-implanted mouse model. In comparison to a previous similar experiment with PTXSXC225, better antitumor activity was observed for the PTXGLC225 conjugate as compared to controls. These results demonstrated the first time use of radioiodinated 3′-OH-PTX for in vivo tracing of a paclitaxel conjugate and application of the resulting information to the design of a therapeutically more useful PTX-MAb linker.