On August 21-26, 1977, two symposia were included in the program of the 10th Annual Meeting of the Society for Invertebrate Pathology held at Michigan State University, East Lansing, Michigan. One was entitled "Invertebrate Models for Biomedical Research" organized by Dr. Thomas C. Cheng, and the second, organized by Dr. Robert S. Anderson, was entitled "Cellular and Humoral Reactions to Disease by Invertebrate Animals." When the final manuscripts of the speakers were received, it became apparent that all of the papers were so closely related that the editors decided that they should be combined and published in a single volume of Comparative Pathobiology under the subtitle of Invertebrate Models for Biomedical Research. This volume is the result. We hope that volume four will provide the reader further insight into the complexity and comprehensiveness of pathobiology. Pathobiology encompasses not only the study of pathologic conditions but also the biology of causative agents and response reactions.