Antibodies to cytoplasmic antigens can be identified with a microscope-based indirect immunofluorescence assay that uses a mouse stomach-kidney substrate. The antibodies are diagnostic markers in chronic active hepatitis, primary biliary cirrhosis, pernicious anemia, and other autoimmune diseases. We describe the development and features of an image-based computer program for teaching medical technologists and other health care workers the proper interpretation of cytoplasmic fluorescence staining patterns. The program, called Cytoplasmic Tutor, is written in Microsoft Visual Basic for Windows and runs on an 80486 microcomputer. it is based on a library of digital images, with key features described by overlays of text. The images were collected and processed with a computer-based fluorescence video microscopy system assembled in our laboratory.