The present study aimed to determine P-glycoprotein expression according to TVT cell morphology in 42 dogs with confirmed TVT, classified into lymphocytoid, plasmocytoid, and mixed. The chemotherapy efficiency was investigated along with its relation to P-glycoprotein expression in tumoral cells, evaluated by immunocytochemistry, considering positive tumors more than 10% stained. Among the samples collected, 50.00% possessed plasmocytoid morphology, 18.63% lymphocytoid, and 31.37% mixed. The plasmocytoid presented greater immunoreactivity to the anti-P-glycoprotein antibody in relation to the lymphocytoid. It was observed that the plasmocytoid was less sensitive to chemotherapy. The cases that had partial clinical response in the lymphocytoid were less compared with those in the plasmocytoid. Therefore, we can infer that the plasmocytoid presents a potential drug resistance. Based on this study, it can be concluded that the plasmocytoid presents a higher frequency of partial clinical response than the lymphocytoid and mixed, possibly through its higher percentage of anti-P-glycoprotein antibody expression.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)