Acute antibody-mediated rejection (AAR) of mouse skin allografts was studied in nine donor-recipient combinations with increasing histoincompatibility ranging from an H-Y to a complete H-2 plus on-H-2 disparity. AAR was induced by the injection of specific alloantiserum along with a heterologous complement on day 7 after grafting. Sera from rabbits, guinea pigs, and from a human volunteer were used as complement sources. The recipients were treated with antilymphocyte serum on days 0, 2, and 4 to postpone cell-mediated rejection. With increasing histoincompatibility the mean survival time of untreated grafts decreased, the in vitro cytotoxic activity of the alloantiserum rose, and AAR could be induced with lower amounts of antiserum. The higher efficiency of rabbit complement compared with guinea pig complement and human complement, that is known to exist in in vitro cytotoxicity, was also found in vivo. Rabbit complement could induce AAR in combination with relatively weak histoincompatibility (H-2K, H-2D, or non H-2 differences), where guinea pig complement and human complement were ineffective. All three complement species elicited AAR if there was a disparity for H-2D plus non-H2 H-2K plus non H-2, H-2, or H-2 plus non-H2. The rules for immunogenicity of the different histocompatibility loci as they have been described for cell-mediated graft destruction also apply to this humorally mediated rejection process.