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.