TSP-1 is a matrix-bound adhesive glycoprotein, which plays a role in tumor cell proliferation and tumor angiogenesis. The purpose of this study was to investigate the effect of TSP-1 on breast tumor cell invasion. Tumor cell invasion assays were performed using a modified Boyden chamber apparatus with collagen-coated membranes. Four breast cell lines were studied in serum-free media: the malignant MDA-MB-231, SKBR-3, and MCF-7 cell lines, and the benign MCF-10A cell line. Invasion was measured as the summation of the number of cells in five representative high power fields (400x) traversing the collagen barrier after a 3-hr incubation period. The effect of an anti-TSP-1 antibody (100 μg/ml) was also evaluated in the malignant cell lines. Statistical analysis was performed by ANOVA and Student's unpaired t test. TSP-1 promoted a dose-dependent increase in invasion as compared to buffer controls in all three malignant cell lines. TSP-1 (100 nM) promoted a greater than five-fold increase over controls in tumor cell invasion for MDA. MB-231, SKBR-3, and MCF-7 cell lines (P < 0.005). TSP-1 had no effect on the invasiveness of the benign cell type MCF-10A. Anti-TSP-1 antibody inhibited TSP-1 promoted invasion in the MDA-MB-231, SKBR-3, and MCF-7 cell lines by 45, 48, and 39%, respectively (P = 0.003, 0.044, 0.047). TSP-1 promotes tumor cell invasion of collagen by breast cancer cells. Therapy designed to inhibit TSP-1 may prevent invasion and metastasis in breast cancer.