The acute phase response in a given individual represents the integrated sum of multiple, separately regulated changes. Although many of these changes commonly occur together in affected individuals, clinical experience indicates that not all of them occur in all individuals, indicating that they must be individually regulated. For example, febrile patients may have normal blood levels of CRP and vice versa, leukocytosis does not always accompany other acute phase phenomena, and many instances of discordance between levels of the various acute phase proteins are seen. Cytokines function as part of a complex regulatory network, a signalling language in which information is conveyed to cells by combinations, and perhaps sequence, of intercellular messenger molecules. The effects of combinations of cytokines are complex. To use a somewhat crude simile, individual cytokines can be thought of as words which bear informational content and which may, on occasion, communicate a complete message. More commonly, however, the actual messages received by cells probably resemble sentences, in which combinations and sequences of words convey information. Currently available data suggest that hepatocytes receive a complex mixture of humoral or paracrine signals during the acute phase response. These are integrated by multiple interacting signal transducing mechanisms to cause finely regulated changes in plasma protein synthesis. Regulation largely occurs by transcriptional control, but post-transcriptional mechanisms, including translational regulation, may also 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 plasma protein changes. Regulation of non-hepatocyte acute phase phenomena has not been delineated as thoroughly, but clearly involves a number of inflammation-associated cytokines.