This invention provides compositions and methods for fixing biological samples, particularly fecal samples used in the diagnosis of parasitic infection. The fixative composition of the present invention includes a zinc salt, an organic acid and water and permits staining of biological samples without use of toxic compounds, such as formaldehyde and mercury-containing compounds and without the use of additives such as polyvinyl alcohol. The fixative composition and methods are compatible with many diagnostic assays, including trichrome stains, iron hematoxlin, ELISA, fluorescent assays, and lateral flow assays.
