The ELF alkaline phosphate substrate can be used to fluorescently label a wide variety of biological targets. This substrate yields a bright, photostable yellow-green fluorescent precipitate at the site of enzymatic activity. ELF labeling can be as much as 40 times as bright and hundreds of times as photostable as labeling with conventional fluorophores and yields signals capable of very fine submicroscopic resolution. Signal development is also extremely rapid, making the signal amplification technology well suited for applications such as RNA in situ hybridization.