The activation of the first component of complement, Cl, initiates the classical complement sequence and plays a critical role in the host immune response. Cl is a complex protein consisting of at least three unique subcomponents, Clq, Clr and Cls held together in a Ca plus 2 dependent complex. The interaction of these subcomponents with each other and with Ca plus 2 is essential for the activation process. The problem of obtaining sufficient human Cl has been overcome by the availability of Cohn Fraction I as an alternative to human plasma. Milligram quantities of the activated Cl subcomponents are now being routinely isolated. Spectroscopic techniques such as ultraviolet difference spectroscopy, fluorescence spectroscopy and nuclear magnetic resonance will be used to investigate the interaction of these activated subcomponents with metal ions and with each other in the presence and absence of metal ions. The metal ions to be studied are Ca plus 2, which is required for Cl biologic activity and Tb plus 3, which has been reported to replace Ca plus 2 and also has fluorescent properties that are altered on protein binding. In addition to these studies, the use of Cohn Fraction I as a source of zymogen Clr and Cls will be investigated.