i) Human cardiac cDNA libraries were constructed and clones encoding human cardiac troponin I (cTnI) isolated. ii) Both the entire <i>cTNI</i> cDNA and a 5'-portion, were expressed in <i>Escherichia coli</i> as fusion products with β-galactosidase. The full-length cDNA was also expressed unfused. iii) The murine monoclonal antibody 29Mu is specific for human and baboon cTnI whereas the 31Mu antibody reacts with cTnI from a range of species. These antibodies might be useful in the imaging of necrotic cardiac tissue. In this study, 31Mu was found to bind to all prepared forms of cTnI antigen, in enzyme-linked immunosorbant assays (ELISAs) and, where tested, in Western blots. Thus, its epitope is localised towards the N-terminus of cTnI. 29Mu bound to the bacterially-produced unfused cTnI but not to the fusion polypeptides or crude bovine cTn. Its ability to bind to human cardiac extracts was related to the method of their preparation, indicating that the epitope of 29Mu shows greater conformational dependency than that of 31Mu. iv) cDNAs encoding the variable domains of 29Mu and 31Mu were cloned and chimaeric antibodies, comprising murine variable and human constant regions produced. Humanised antibodies, in which only the antigen-binding sites were of murine origin, were also produced. Such recombinant antibodies would be expected to exhibit reduced immunogenicity in man. v) Neither the chimaeric nor humanised antibody versions of 29Mu bound cTnI. Chimaerised 31Mu reacted with all forms of cTnI but did not show complete equivalence to 31Mu. An antibody containing the humanised 31 kappa chain and the chimaeric heavy chain was reactive to all forms cTnI in ELISAs but its efficiency of binding, relative to that of the chimaeric antibody, was dependent upon the antigen source. Humanised heavy chains were produced utilising two different human frameworks and the framework, showing closer homology to the 31Mu variable domain, supported antigen binding with fewer murine residue substitutions. However, both successful humanised 31 antibodies showed some cross-reactivity.