Cytokines, i.e., regulatory proteins derived primarily (but not exclus
ively) from cells of the immune system, are receiving increasing atten
tion for their influences on physiological processes. This paper outli
nes several of the unique characteristics of cytokines and discusses t
he pitfalls encountered when measuring them in biological fluids. At p
resent, each available assay has a combination of advantages and drawb
acks; therefore, investigators must be aware of the trade-offs and cho
ose the assay that best addresses their needs. The factors that affect
cytokine measurement also influence cytokine activity in vivo; thus t
hey are important from a physiological as well as methodological stand
point. Moreover, the absolute concentration of a single cytokine is pr
obably less important than the balance between that cytokine and its n
atural antagonists.