There are many unanswered questions regarding the molecular nature of antibodies, components of complement, and other substances which participate in the immune response. The list of substances which need to be analyzed chemically is increasing. Plasma cell products, of course, have long been of great interest because the most prevalent ones are immunoglobulins. Other cell types, however, are the source of the broad spectrum of additional substances which classically fall into the sanctum of the molecular immunologist. It is these substances, and especially those more recently discovered, which are responsible for the broadening investigative interests of immunologists. In this volume we have provided you with descriptions of research being done with immunoglobulins and with complement. Additionally, we have included two reports that deal with molecules which are among the more recent acquisitions of the molecular immunologist. The components of complement are known to react in a cascading manner which results in the lysis of cellular antigens. The first step in the classical pathway requires the activation of CI by the antibody-antigen aggregates. This volume of Contemporary Topics in Molecular Immunology begins with the report of Reid and Porter which describes their investigation of the mechanism of activation of C I. Their descriptions of C I q and of the reaction of C I with immunoglobulins are especially intriguing. It is clearly apparent from their report that activation of the components of complement is a complex phenome­ non.