Many proteins containing domains of a cysteine-rich repeated motif, such as epidermal growth factor (EGF), have been reported. Here we report strong similarity between the amino acid sequence of a plant toxin--i.e., purothionin and its homologues--and with those of a domain found in mammalian pore-forming cytoplasmic proteins: components of complement and perforin of cytotoxic T-lymphocytes or natural killer-like cytotoxic cells. These similar sequences were found to be identical to the so-called EGF-like cysteine-rich repeated motif itself. Electron-microscopic observations indicated that, like complement and perforin, purothionin forms pores in the cytoplasmic membrane of target cells, resulting in their death within a few hours. On the basis of these sequence comparisons and physiological functions, we propose a scheme for the evolution of proteins containing modules of the cysteine-rich repeat motif.