According to the difference of molecular masses estimated by SDS-polyacrylamide gel electrophoresis, calpastatins are classified into two types, i.e. muscle type (110 kDa) and erythrocyte type (70 kDa). Muscle type calpastatin contains four internally repetitive sequences (Domains 1-4) and one nonhomologous sequence on the amino-terminal side (Domain L), whereas erythrocyte type lacks Domains L and 1. By immuno-blot analysis, chicken erythrocytes, nucleated cells, were found to contain muscle type calpastatin. In avian erythrocytes, diminution of the calpastatin molecule as in mammalian erythrocytes was not observed.