The disclosure provides a method for 
immunotherapy of a subject afflicted with 
cancer, comprises administering to the subject a composition comprising a therapeutically effective amount of an 
antibody that inhibits signaling from the PD-1 / PD-L1 signaling pathway. This disclosure also provides a method for 
immunotherapy of a subject afflicted with 
cancer comprising selecting a subject that is a suitable candidate for 
immunotherapy based on an assessment that the proportion of cells in a test 
tissue sample from the subject that express PD-L1 on the 
cell surface exceeds a predetermined threshold level, and administering a therapeutically effective amount of an anti-PD-1 
antibody to the selected subject. The invention additionally provides rabbit mAbs that bind specifically to a 
cell surface-expressed PD-L1 
antigen in a FFPE 
tissue sample, and an automated IHC method for assessing 
cell surface expression in FFPE tissues using the provided anti-PD-L1 Abs.