The First International Workshop was held at Carry-le-Rouet, France, September 14-16, 1981. The Workshop brought together for the first time leading investigators in each of the principal areas of cell-mediated cytotoxicity, as well as experts in the area of complement-mediated cytoxicity. Formal research presentations were held to a minimum, the emphasis being on open discussion of current knowledge about mechanisms of cytoxicity in each of the systems under consideration. The major objectives of the Workshop were 1) to compare and integrate what is known about the mechanism(s) of cytoxicity in each system; 2) to determine whether, on the basis of information in hand, it seems likely that the mechanisms of cytotoxicity in the various systems are the same or are unique; and 3) to stimulate thinking about new approaches to elucidating the fundamental mechanisms by which certain cells are able to kill other cells. This volume is intended as something more than a simple report or record of the Workshop. Various participants were asked to write either a review on a given topic, or a more detailed specific account of relevant current research. The mass of formal presentation in this volume thus far exceeds the amount of formal presentation that actually occurred. Each author has been encouraged to engage in a bit more speculation about possible mechanisms than might be appropriate for a standard research journal. Moreover, because most of the time in the various sessions was spent in open discussion we have tried to capture some of that flavor by appending portions of these discussions after papers, where appropriate. This may convey some impression of how at least some of us defend our present-day uncertainties.