Fluorocillin™ Green 345/530 β-lactamase substrate was developed as a precipitating dye generally compatible with a variety of β-lactamase enzymes and corresponding antibody conjugates. Scientists at Molecular Probes have demonstrated the utility of this reagent in immunohistochemistry applications, and it may also prove useful as a marker for endogenous β-lactamase activity in prokaryotes, possibly detected by microscopy or even flow cytometry. It is important to note, however, that in its precipitated form, Fluorocillin™ Green 345/530 β-lactamase substrate produces a crystal size incompatible with immunocytochemical analysis. This form of β-lactamase substrate is not compatible with TEM-1 β-lactamase and its conjugates, but can, for example, be cleaved by P99 β-lactamase. It is supplied as a dry powder in a unit size of 5 mg.