A promising approach to increase the specificity of photosensitizers used in photodynamic therapy has been through conjugation to monoclonal antibodies (MAb) directed against tumour-associated antigens. Many of the conjugations performed to date have relied on the activated ester method, which can lead to impure conjugate preparations and antibody crosslinking. Here, we report the development of photosensitizer-MAb conjugates utilising two porphyrin isothiocyanates. The presence of a single reactive isothiocyanate allowed facile conjugation to MAb FSP 77 and 17.1A directed against internalizing antigens, and MAb 35A7 that binds to a non-internalizing antigen. The photosensitizer-MAb conjugates substituted with 1-3 mol of photosensitizer were characterised in vitro. No appreciable loss of immunoreactivity was observed and binding specificity was comparable to that of the unconjugated MAb. Substitution with photosensitizer had a minimal effect on antibody biodistribution in vivo for the majority of the conjugates, although a decreased serum half-life was observed using a cationic photosensitizer at the higher loading ratios. Tumour-to-normal tissue ratios as high as 33.5 were observed using MAb 35A7 conjugates. The internalizing conjugate showed a higher level of phototoxicity as compared with the non-internalizing reagent, using a cell line engineered to express both target antigens. These data demonstrate the applicability of the isothiocyanate group for the development of high-quality conjugates, and the use of internalizing MAb to significantly increase the photodynamic efficiency of conjugates during photoimmunotherapy.