The amino acid sequences of the 51% different horseradish peroxidase HRP C and turnip peroxidase TP 7 have previously been completed by us, but the three-dimensional structures are unknown. Recently the amino acid sequence and the crystal structure of yeast cytochrome c peroxidase have appeared. The three known apoperoxidases consist of 300 +/- 8 amino acid residues. The sequences have now been aligned and show 18% and 16% identity only, between the yeast peroxidase and plant peroxidase HRP C and TP 7, respectively. We show that different structural tests all support similar protein folds in plant peroxidases and yeast peroxidase and, therefore, a common evolutionary origin. The following tests support this thesis: (a) predicted helices in the plant peroxidases follow the complex pattern observed in the crystal structure of cytochrome c peroxidase; (b) their hydropathic profiles are similar and agree with observed buried and exposed peptide chain in cytochrome c peroxidase; (c) half-cystines which are distant in the amino acid sequence of plant peroxidases become spatial neighbours when fitted into the cytochrome c peroxidase model; (d) the two-domain structure proposed from limited proteolysis of apoperoxidase HRP C is observed in the crystal structure of cytochrome c peroxidase. The similarities and differences of the plant and yeast peroxidases and the reactive side chains of a plant peroxidase active site are described. The characteristics of Ca2+-binding sequences, derived from several superfamilies, are applied to predict the Ca2+-binding sequences in plant peroxidases.