Colorimetric and fluorometric methods are disclosed for the rapid, accurate, selective, and inexpensive detection of homocysteine, or of homocysteine and cysteine, or of cysteine. The methods may be employed with materials that are readily available commercially. The novel methods are selective for homocysteine, for cysteine, or for total homocysteine and cysteine, and do not cross-react substantially with chemically-related species such as glutathione. The homocysteine-selective method does not have substantial cross-reactivity to the very closely related species cysteine. The cysteine-selective method does not have substantial cross-reactivity to the very closely related species homocysteine. The methods may be used, for example, in a direct assay of human blood plasma for homocysteine levels.