We report an object-oriented data handler and supplementary tools for the development of molecular genetics application software for various sequence analyses. Our data handler has a flexible and expandable format that supports the most common data types for molecular genetic software. New data types can be constructed in an object-oriented manner from the basic units. The data handler includes an object library, a format-converting program and a viewer that can visualize simultaneously the data contained in several files to construct a general picture from separate data. This software has been implemented on an IBM PC-compatible personal computer.