Receptor-specific antibodies can both prevent ligand–receptor interaction and initiate receptor signaling. Previously we generated monoclonal antibody 8E8 (mAb 8E8) against protease-activated receptor type 3 (PAR3) which inhibited proliferation of B cell hybridoma. Here we used mAb 8E8 and PAR1-specific polyclonal antibody to reveal the functions and cooperating partners of PAR3 in endothelial cells and in B lymphocytes. MAb 8E8 or PAR1 agonist peptide stimulated IL-6 and IL-8 production and VCAM-1 expression in HPMEC-ST1.6R cells. PAR1 antibody stimulated only VCAM-1 expression, while ICAM-1 expression was stimulated with mAB 8E8 or PAR3 peptide. MAb 8E8 stimulated weak mitogenic response, while PAR1 antibody inhibited it in normal but not in malignant B lymphocytes. Sandwich ELISA assay demonstrated the interaction of PAR3 with PAR1 in malignant cell lines and with IgM in normal B lymphocytes. It is concluded that PAR3 cooperates with PAR1 to mediate the effect of thrombin on cytokine production and VCAM-1 expression in endothelial cells and on cell proliferation in malignant B cells. ICAM-1 expression in endothelial cells requires PAR3 without PAR1. The inhibitory effect of thrombin in normal B lymphocytes is mediated by PAR1 alone, while mitogenic and pro-survival signaling in B lymphocytes is provided through PAR3 in cooperation with BCR.