Feline infectious peritonitis MESHD peritonitis HP (FIP) is associated with mutations in the feline coronavirus (FCoV) genome that are thought to convert the subclinical feline enteric coronavirus (FECV) into the lethal feline infectious peritonitis MESHD peritonitis HP virus (FIPV). A key feature of FIPV, not shared with FECV, is the productive infection MESHD of macrophages. Therefore mutations in proteins that govern cell tropism, such as the spike glycoprotein, may play an important role in FIP progression. In a prior study, involving a limited number of samples, we have shown an association of FIP with mutations in the protease cleavage-activation site located between the receptor-binding and fusion domains of the FCoV spike (S1/S2). Here, we extend these studies to investigate a larger sample set and to obtain a more refined analysis of the mutations at this S1/S2 site. Our larger data set more clearly shows that the mutations acquired by FIPV at S1/S2 are also accompanied by additional mutations at a second protease cleavage-activation site located in the fusion domain (S2'), adjacent to the viral fusion peptide. Overall, our data indicate a pattern of mutations across the two protease recognition sites that results in substitutions, and/or altered recognition, of critical basic/polar amino acid residues needed for virus activation in the enteric tract. Typically, FIPVs have substitutions of non-polar, aliphatic or aromatic residues in the protease recognition sites. These changes likely modulate the proteolytic activation of the virus and its ability to productively infect macrophages in vivo.