This invention relates to a method of assaying pyrrole-containing biological compounds and chemical compositions that can be used in the method. The method involves contacting a biological compound with one of: a) a bound or bindable derivatizing agent which forms a reaction product with the biological compound, followed by exposure to a detectable molecule which forms a complex with the reaction product; or b) a derivatizing agent which forms a reaction product with the biological compound, followed by exposure to a bound binding agent specific to the biological compound in the reaction product; or c) a binding agent specific to the biological compound, followed by exposure to a derivatizing agent which forms a reaction product with the biological compound, and determining the amount of bound biological compound. There is also provided a method of preparing an antigen.