comparison of the effect of various antisera and cobra venom factor on inflammatory reactions in guineapig skin ii the arthus reaction and the local shwartzman reaction the ability of antisera to guineapig c3 to inhibit the arthus and local shwartzman reactions was studied they were found to reduce the nonhaemorrhagic component of the active and reversed passive arthus reactions and to delay the appearance of the haemorrhage in the active arthus reaction cobra venom factor however had no effect on the nonhaemorrhagic components of these reactions and only delayed the appearance of the haemorrhage of the active arthus reaction there appeared to be a correlation between the serum complement level and the time taken for the haemorrhage to appear and between the circulating platelet count and the extent of the nonhaemorrhagic oedematous component of the reaction the haemorrhagic component of the local shwartzman reaction was not affected by decomplementation with cobra venom factor the ability of the antisera to inhibit the haemorrhage of the shwartzman reaction was not dependent on lowering the serum complement titre however the haemorrhage was inhibited if the circulating platelet count was also reduced to very low numbers antiserum to zymosan alone had the same effect as antibeta1cbeta1a globulin zymosan in blocking the reaction although it did not alter the complement levels or the platelet counts the possibility of an immunological crossreactivity between zymosan and endotoxin in this action is discussed