The cattle tick Boophilus microplus, a blood-feeding ectoparasite, is a major restraint to milk and beef profitability in many countries of the world. In hematophagous parasites saliva there are some molecules to facilitate feeding like anti-coagulants and immunomodulators. These molecules are strange elements inside the host, witch develop an immune response against these invasors bodies. IgG are produced by the host T-cells for that purpose. In this work we investigated the hypothesis that B. microplus saliva inactivates IgG. IgG was purified from cattle serum by affinity chromatography (G-protein Sepharose Hitrap 5/5 ml) and quantified in a spectrophotometer at 280 nm. Fully engorged female saliva was incubated with purified IgG at different conditions: pH 7.0 at 37ºC during 1 h, 2 h, 4 h, 8 h and at room temperature overnight. Samples were analyzed for IgG fragments by SDS-PAGE. Tick saliva pH was measured and presented a basic pH 10.0 value. Presence of proteolytic activity in saliva was also verified by zymograms using 0.1% IgG-poliacrylamide copolymerized gels. Gels were incubated at pH 8.0 and pH 4.5, and stained with coomassie brilliant blue R-250. In all experiments no IgG fragments were detected. Also, we searched for the presence of other proteolytic activity in the saliva, using synthetic substrates. Since the salivary gland is rich in a cathepsin L like enzyme, its activity was tested using the fluorogenic substrate N-Cbz-Phe-Arg-MCA (1.4 mM), in acetate buffer pH 3.5 10 mM DTT, at 37ºC. The kinetic of hydrolysis was analyzed in a microplate fluorimeter (fmax) at 370_ex-460_em nm. Trypsin activity assays were done in Tris-HCl buffer pH 7.0 and 10.0 using Bz-Arg-PNA (300 mM) as substrate. The reaction course was followed using a microplate spectrophotometer (Spectra max) at 405 nm. Also, no activity was detected. So, we conclude B. micoplus saliva (i) does not degrade bovine IgG; (ii) does not have trypsin like activity and (iii) the salivary gland does not secrete the catephepsin-like enzyme into the saliva.