A new porphyrin isolated from calf and foal meconium has been characterized as a tris-(2-carboxyethyl)-mono-(2-carboxyvinyl)-tetramethyl porphin, that is as an analogue of coproporphyrin in which one propionate group has been replaced by an acrylate group. The porphyrin is mainly a type III isomer and its possible significance in haem biosynthesis is discussed.