The aim of this work was to determine levels of carcinoembryonic antigen (CEA) and cancer antigen (CA 15-3) in the blood serum of 45 bitches. A modified procedure was used to determine the CEA and CA 15-3 markers with the human kits using the radioimmunoassay method. Samples collected from extirpated tumour of mammary glands were histologically processed and classified as per WHO guidelines. The average age of animals with tumour was 10.00 ± 2.2 years; for healthy bitches average age was 4.2 ± 3.2 years. Values of CEA and CA 15-3 were considered positive, if they exceeded 0.23 ng mL−1 and 7 IU mL−1, respectively. Average levels of CEA in the tumour group were 0.25 ± 0.06 versus 0.20 ± 0.03 in healthy bitches (P = 0.0001). The average CA 15-3 value in bitches with tumour was 8.58 ± 1.27 versus 5.14 ± 1.34 in healthy animals (P < 0.0001).