Antibiotics can alter the host's reaction to an infection (itself an immunomodulating event) in various ways. Indirect actions involve killing of bacteria, changing the intestinal flora, intrinsic antigenicity and preventing bacteria making virulence factors. Direct actions are upon phagocytic function, chemotaxis and lymphocyte activities. Immunomodulation can be positive ("pro-host") or negative, and can be quantitated by means of the parameter "immune index". Among the cephalosporins, cefodizime shows the greatest positive immunomodulating activity, due to the unique nature of the 3-sidechain. Cefotaxime has an immunodepressing effect in vitro. The oral cephalosporin cefaclor appears to have a beneficial effect on polymorph function. While immunomodulation by antibiotics may appear marked in in vitro and ex vivo experiments, and in animal models, this phenomenon does not appear to have decisive effects therapeutically.