Compounds which are substrates for mammalian enzymes, especially proteolytic and peptidase enzymes, are disclosed. These compounds, in general, have the general formula X-T, W-Y or X-(Pep)-Y, wherein X is a radiolabelled benzoyl or hydroxyphenylpropanoyl group, T is the --NH residue of a known chromogenic or fluorogenic substrate comprising at least one amino acid group which may be substituted with other organic groups, Y is a radiolabelled benzylamine group, W is the ##STR1## residue of a known chromogenic or fluorogenic substrate comprising at least one amino acid group which may be substituted with other organic groups, and (Pep) is a difunctional or polyfunctional residue from a known chromogenic or fluorogenic substrate comprising at least two amino acid groups in peptide linkage, which residue may be substituted with other organic groups. These substrates exhibit many advantages over the chromogenic and fluorogenic analogs, including greater sensitivity, surprising and unpredictable affinity and specificity, and the like.