Abstract: Novel pheophorbide derivatives of the general formula: ##STR1## [wherein Z is O or NH; n is an integer of 1 to 6; Y is NR'R" or ##STR2## wherein R', R" and R'" are the same or different or each represents a C.sub.1 to C.sub.4 lower alkyl group; X.sup.- is a halogen or organic acid ion); R.sub.1 is an ethenyl group; C.sub.1 to C.sub.4 lower alkyl group or ##STR3## (wherein m is an integer of 0 to 6; R.sub.6 is H or a C.sub.1 to C.sub.4 lower alkyl group); R.sub.2 is CH.sub.3, CHO or CH.sub.2 OH; either of R.sub.3 and R.sub.4 is H, with the other being OH, or both of them combine to represent .dbd.O; R.sub.5 is H or CO.sub.2 CH.sub.3) are provided.When administered to a cancer-carrying animal, the derivatives accumulate specifically in cancer tissues and emit characteristic fluorescent spectrum under irradiation of light, which enables one to detect the cancer tissues.