Erylysin A (EryA), an aegerolysin protein produced by the edible king oyster mushroom (Pleurotus eryngii) interacts strongly with an invertebrate-specific membrane sphingolipid ceramide phosphoethanolamine. Recently, a fluorescently fused variant of EryA was shown to bind to artificial and bacterial lipid membranes containing cardiolipin (CL). This tetra-acylated glycerophospholipid, present in bacteria and in inner mitochondrial membranes of eukaryotic cells, was shown to be externalized to the plasma membrane surface during the process of apoptosis. In this work, we evaluated the interaction of EryA-mCherry with CL-containing artificial lipid vesicles and with mammalian cells undergoing apoptosis, and compared its binding affinity and specificity to that of the well-established apoptosis marker, annexin V-FITC. Our results show that, in contrast to annexin V-FITC which binds different negatively charged glycerophospholipids, EryA-mCherry specifically recognizes and binds CL in artificial membrane systems. Experiments using mammalian cells showed the ability of EryA-mCherry to selectively label the surface of apoptotic cells, exhibiting the same labeling pattern as anti-CL antibodies. Our data suggest that EryA-mCherry might be used as a marker of early apoptosis, as well as a marker of CL in biological and artificial lipid membranes.