Provided herein are methods of treating prolactin receptor positive breast cancer using an antibody drug conjugate (ADC) comprising an anti-PRLR antibody or antigen-binding fragment thereof conjugated to a cytotoxic agent. In certain embodiments, the anti-PRLR antibody or antigen-binding fragment thereof is conjugated to maytansinoid. In certain embodiments, the method of treating PRLR positive breast cancer includes administering the ADC in combination with one or more chemo therapeutic agents.