Cytokines appear to function as part of a complex regulatory network, a signaling language in which informational content resides in the combinations, and perhaps sequence, of cytokines and other extracellular messenger molecules received by a cell. The effects of combinations of cytokines are complex and often differ from their in vitro effects or when administered by themselves. (From this perspective, to use a somewhat crude simile, individual cytokines can be thought of as words which bear informational content. Although individual cytokines may, on occasion, communicate a complete message, more commonly the actual messages received by cells probably resemble sentences, in which it is the combination and sequence of words which convey information.) Currently available data suggest an in vivo scenario in the acute phase response in which the hepatocyte receives a complex mixture of humoral or paracrine signals which are integrated by multiple interacting post-receptor and gene regulatory mechanisms to cause finely regulated changes in plasma protein synthesis. Regulation often occurs by transcriptional control, but post-transcriptional mechanisms, including translational regulation, may participate. Both the extracellular and intracellular mechanisms that mediate the response of the hepatocyte to inflammatory stimuli appear to be highly complex and involve multiple overlapping, concurrent, and parallel pathways. Enough is known at present to conclude that IL-6 is a major participant in these changes in man. Regulation of non-hepatocyte acute phase phenomena has not been delineated as thoroughly, but clearly involves a number of cytokines.