A 4-h bout of exercise induces immunomodulatory effects. Peripheral blood was withdrawn before, and at 4, 8 and 24 h after the start of exercise, RNA from the unfractionated white blood cells was analyzed using Agilent human 44 K microarray The expression profiles were sorted into seven clusters based on their unique time-dependent kinetics In a separate experiment, cell-specific markers were collected and compared among the members in each Cluster Two Clusters were assigned as representing neutrophils, one as NK cells, and another mostly as T cells Three Clusters seemed to be mixtures of several cell types Extension of this approach to other systems is discussed. (C) 2009 Elsevier Inc. All rights reserved.