Ticks are obligatory blood-sucking arthropods and some species are very important vectors of both human and animal diseases. In order to study the immune response of ticks against pathogens, we are using a culture of embryonic cells (BME 26) of Boophilus microplus (Exp. Appl. Acarol. 1989, 7 :219) as a model. We showed that these cells present glycogen inclusions, numerous mitochondria and vesicles with heterogeneous electron densities dispersed through the cytoplasm. Moreover, the uptake of palladium meso-porphyrin (Pd-mP) and hemoglobin by these vesicles indicate their involvement in the endocytic pathway. One cDNA library was constructed and 898 unique sequences were obtained. Among them, 558 sequences did not reveal similarity to sequences from public databases (Esteves et al., XXI Annual Meeting of the Brazilian Society of Protozoology, 2005). We are concluding the manual annotation of the remaining 340 sequences and several transcripts related to the immune system have already been identified. The expression of the genes that encode the antimicrobial peptides microplusin and defensin as well as the phagocytic activity exhibited by cells suggest their role immune system. In an attempt to verify the localization of these antimicrobial peptides, we submitted the cellular extract and culture medium to RP-HPLC. However, no antimicrobial activity was detected. On the other hand, we observed microplusin in these cells by imunofluorescence, suggesting that these antimicrobial peptides are present in low amount. We are investigating the expression profile of the genes related to the antimicrobial peptides upon challenge with bacteria. Besides the relevance of the antimicrobial transcripts will be also evaluated through the silencing of their expression by RNAi in a near future.