Inhibition of tumor growth by thrombospondin-1 (TSP1) is generally attributed to its anti-angiogenic activity, but effects on tumor immunity should also be considered. We show that over-expression of TSP1 in melanoma cells increases macrophage recruitment into xenograft tumors grown in nude or beige/nude mice. In vitro, TSP1 acutely induces expression of plasminogen activator inhibitor-1 (PAI-1) by monocytic cells, suggesting that TSP1-induced macrophage recruitment is at least partially mediated by PAI-1. Tumor-associated macrophages can either promote or limit tumor progression. The percentage of M1 polarized macrophages expressing inducible nitric oxide synthase is increased in TSP1-expressing tumors. Furthermore, soluble TSP1 stimulates killing of breast carcinoma and melanoma cells by interferon-γ-differentiated U937 cells in vitro via release of reactive oxygen species. TSP1 causes a significant increase in phorbol ester-mediated superoxide generation from differentiated monocytes by interaction with α6β1 integrin through its N-terminal region. The N-terminal domain of thrombospondin-2 also stimulates monocyte superoxide production. Extracellular calcium is required for the TSP1-induced macrophage respiratory burst. Thus, TSP1 may play an important role in anti-tumor immunity by enhancing recruitment and activation of M1 tumor-associated macrophages, which provides an additional selective pressure for loss of TSP1 and thrombospondin-2 expression during tumor progression.