Human C-reactive protein (CRP) is an acute phase protein, which harbours both host defence and scavenging properties. In this study, we obtained two new crystal forms of CRP, where CRP forms a symmetric, staggered dimer of pentamers. In one of these structures, obtained in the presence of HIV-1 Tat protein, this dimer of pentamers is stabilized by two zinc ions trapped within a cleft of the effector face of CRP. These two decameric interfaces involve complementary surfaces of CRP pentamers and bury a large area of ~2000 Å(2) per pentamer, suggesting a biological role of this interface. These two novel decameric interfaces and the involvement of zinc might have important consequences in the understanding of CRP biological functions.