We have always experienced difficulties in the past in treating cells sensitized with blood group antibody with the first component of complement so that they became strongly active following such treatment. Such cells, prepared as described by other workers (Harboe, Müller-Eberhard, Fudenberg, Polley and Mollison, 1963) have always proved to be insufficiently active for our experiments.

We therefore explored a number of alternative methods, and found that a pellet obtained by ultracentrifugation of human serum (Naff, Pensky and Lepow, 1964) could be used for this purpose. The object of this paper is to describe the treatment of cells sensitized with blood group antibodies with this pellet and to describe the reactions of the cells so prepared.