In contrast to previous reports, L-rhamnose as well as L-fucose is capable of inhibiting guinea pig migration inhibition factor (MIF) activity. These sugars also inhibit the activity of macrophage chemotactic and neutrophil chemotactic factors. Thus, fucose is not a specific inhibitor of MIF, nor is its activity confined to that lymphokine. Preincubation of L-fucose or L-rhamnose with the lymphokines was required for inhibition of chemotactic activity. This finding, as well as the results of dose-response experiments, suggest that the mechanism of chemotactic inhibition is similar to the mechanism of inhibition of MIF by monosaccharides; i.e., that the lymphokines possess receptor sites that recognize cell sites bearing a structural or conformational similarity to these sugars. In contrast, L-xylose, the only other sugar found inhibitory in the present study, appeared to exert its activity by a direct effect on the target cells themselves rather than by interaction with the mediator. Dose-response studies demonstrate that the effect of L-fucose on inhibition of either lymphokine activity is inversely proportional to the amount of lymphokine activity initially present. A major finding that emerged from the present study is that the monosaccharide inhibition of inflammatory mediator activity appears to be confined to either lymphokines themselves or cytokines (lymphokine-like factors derived from nonlymphoid cells). Three other biologic mediators with chemotactic activity, fragments from trypsin-treated C3 and C5, and bacterial factor, could not be inhibited by either L-fucose or L-rhamnose. The apparent inhibitory effect of L-xylose for bacterial factor activity does not negate this conclusion, since as stated above, this sugar appears to interact with the target cell rather than the mediator. Aside from theoretical considerations, this finding may be of considerable practical importance since it could provide a simple way to distinguish different sources of chemotactic activity in complex biologic mixtures such as serum or tissue extracts. Based upon current data and the results of previous studies, we have postulated that the inflammatory mediators (lymphokines) have two binding sites, one to allow for recognition of the appropriate target cells, and the other which determines the specific biologic activity.