This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Regarding AIDS The undecapeptide substance P (SP) is a member of the tachykinin family of neurotransmitters which has a pivotal role in the regulation of inflammatory and immune responses. The demonstration of elevated levels of plasma SP in a number of immune disorders, including rheumatoid arthritis and the acquired immune deficiency syndrome, has led to increased interest in mechanisms of action involving key effector cells of the immune system and components of the neurokinin pathways. There have been several investigations into the interaction of SP with its preferred receptor (NK1-R), as well as, studies directed toward the development of several SP antagonists as potential immunoregulatory agents. Central to such studies is the accurate measurement of substance P in fluids. There is variability in the reported levels in humans of serum and plasma-derived SP in both healthy and diseased states. This study was initiated in order to identify sources of variability by the comparative evaluation of the influence of sample preparation and analytical detection methods on the measurement of serum and plasma-derived SP. The results indicate that sample preparation (peptide extraction vs no extraction) and choice of analytical method for SP quantitation can yield significantly different values and may contribute to the variability observed in the literature. The results further emphasize the need for careful consideration in the selection of methods used for SP quantitation, as well as caution in the interpretation and comparison of data reported in the literature.