ABSTRACT The objective of the present study was to analyze the serum levels of the tumor marker Ca15.3 in healthy bitches and those with mammary neoplasms, correlating results with tumor type, clinical staging, time until presentation, and presence of ulceration and vascularization. For the study, 30 bitches with mammary tumors and 30 healthy bitches (control group) were selected. Histopathology was performed for identification of tumor type, and blood was collected for measurement of serum concentration of the marker via the chemiluminescence method using a commercial kit. A higher frequency of malignant neoplasms was observed (76.7%), with a higher quantity of carcinoma in mixed tumor (26.7%). Regarding serum concentration of the marker Ca15.3, there was no difference in serum values when comparing the means from bitches with neoplasia and healthy bitches, nor when comparing the other characteristics. The majority of results for serum concentration of Ca15.3, whether in bitches with neoplasia or in healthy bitches, was zero. It is concluded that the measurement of the marker Ca15.3 using the chemiluminescence method and commercial kits for humans did not offer significant results that would make this method or this marker a useful tool for patient monitoring and evaluation of the prognosis of bitches with mammary neoplasms.