Abstract: Phorbine derivatives having high potency and light absorptivity in the 650 to 780 nanometer range are useful in cancer diagnosis and therapy, especially photodynamic therapy.

Abstract: A method for intercepting an activated species of dioxygen by administering an N-substituted pyrrole derivative having the formula: ##STR1## in which: R is an amino acid derivative, a non-protein peptide, an amino sugar processing a free primary amino group, a purine bases which is substituted, at position 9 by alkyl groups, or a pyrimidine bases possessing a free primary amino group, wherein the pyrimidine bases are substituted by at least one SH group; R.sub.1 represents one or more substituents, identical or different and are a hydrogen, an alkyl radical containing from 1 to 6 carbon atoms, a primary amino radical, a secondary amino radical NHB, or a tertiary amino radical--N(B,B'), wherein B and B', identical or different represent an alkyl group of 1 to 6 carbon atoms, a SH or SB radical, wherein B is defined above, a --CH.sub.