The photodynamic therapy is highly promising for its dual specificity, namely, for the site of accumulation of the phototoxic substance and for the site of action of laser. A drawback of the PDT technique is a low selectivity of the photosensitizers accumulation in tumor tissue. Therapeutic doses of photosensitizers are fairly high, that leads to unfavorable effects (toxicoses, lesions of normal skin, prolonged rehabilitation). This drawback can be overcome is photoimmunotoxins are used, i.e. conjugates of photosensitizers with antibodies to specific tumor markers. We tried to introduce a simple model for estimation of singlet oxygen effect on cells for the case of free photosensitizers, presented in outside solution, and in case of conjugated dye, bound by cell surface receptors. To achieve the same effect the required concentration of conjugated photosensitizer can be significantly decreased (50 to 100 fold) as compared with free dye. Methods for porphyrins and different phthalocyanines and naphthalocyanines conjugation with protein were developed. The use of bacteriophage T4 as a model for studying the action of photoimmunotoxines on virus, unambiguously demonstrated the applicability of this approach for targeted destruction of viral particles.