Many physiological, biochemical, and behavioral processes are under circadian regulation, which is generated by an internal timekeeping mechanism referred to as biological clock. The regulators of circadian rhythm in human plasma have not been completely elucidated. Here we demonstrated that the isolated protein from human plasma, which down-regulated expression level of cryl in Jurkat cells, was apoprotein H (ApoH). Further, mRNA expression level of ApoH indicated circadian rhythm in Jurkat cells. The concentration of ApoH was subject to circadian rhythm in human plasma. These experimental results suggested that ApoH may be one of the members of the regulator of circadian rhythm and that ApoH expression level was also dependent on circadian rhythm in Jurkat cells and in human plasma. (C) 2007 Elsevier Inc. All rights reserved.