The aim of the study was to investigate the effects of chitosan oligosaccharide (COS) on some hematological parameters, and define the percentage of alpha-naphthyl acetate esterase (ANAE) positive lymphocytes in rats that had been exposed to cadmium (Cd). Thirty-two (n = 32) male Wistar albino rats were randomly divided into four groups as the control (C), chitosan oligosaccharide (COS), cadmium (Cd), and Cd + COS (CdCOS) groups. Blood samples were collected to assess erythrocytes (RBC), leukocytes (WBC), hemoglobin levels (HGB), hematocrit values (HCT), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), mean corpuscular volume (MCV), platelets (PLT), and alpha-naphthyl acetate esterase (ANAE) positive lymphocytes. The number of WBCs significantly increased in the Cd group compared to the C, CdCOS, and COS groups (P<0.05). Although the lymphocyte count decreased significantly in the Cd group (P<0.05), it was ameliorated by COS treatment in the CdCOS group compared to the Cd group (P<0.05). Also, the percentage of peripheral blood ANAE positive lymphocytes decreased significantly in the CdCOS and Cd groups (P<0.05). COS exhibits a partially protective effect on some hematological characteristics, apart from the percentage of ANAE positive lymphocyte in cases of chronic Cd exposure.