Provided is a monoclonal antibody specifically reactive with a C-terminal neo-epitope of PIIINP comprised in a C-terminal amino acid sequence CPTGXQNYSP-COOH (SEQ ID NO: 4) in which X is Gly or Pro, and where the monoclonal antibody does not recognize or bind an elongated version of the C-terminal amino acid sequence CPTGXQNYSPQZ-COOH (SEQ ID NO: 5), in which Z is absent or is one or more amino acids of the sequence of collagen type III. Also provided is a method of immunoassay for detecting in a biological sample the C-terminal neo-epitope of PIIINP generated by N-protease cleavage of intact type III procollagen, by contacting the sample with the monoclonal antibody, and determining the amount of binding of the antibody.