It is thought that Trypanosoma cruzi, the protozoan that causes Chagas' disease, modulates the extracellular matrix network to facilitate infection of human cells. However, direct evidence to document this phenomenon is lacking. Here we show that the T. cruzi gp83 ligand, a cell surface trans-sialidase-like molecule that the parasite uses to attach to host cells, increases the level of laminin gamma-1 transcript and its expression in mammalian cells, leading to an increase in cellular infection. Stable RNA interference (RNAi) with host cell laminin gamma-1 knocks down the levels of laminin gamma-1 transcript and protein expression in mammalian cells, causing a dramatic reduction in cellular infection by T. cruzi. Thus, host laminin gamma-1, which is regulated by the parasite, plays a crucial role in the early process of infection. This is the first report showing that knocking down the expression of a human gene by RNAi inhibits the infection of an intracellular parasite.