Novel C-13.sup.2-COXR.sub.1, C-17.sup.2-COXR.sub.2 and C-13.sup.2-COXR.sub.1, C-17.sup.2-COXR.sub.1 derivatives of chlorophyll and bacteriochlorophyll compounds are provided wherein X is O, S or N and R.sub.1 and R.sub.2, the same or different, may be an optionally substituted hydrocarbyl, amino acid, peptide, protein or saccharide radical. The compounds are for use in photodynamic therapy (PDT), for diagnosis of tumors, and for killing cells and infectious agents such as bacteria and virus, both in biological products and in living tissues.