A portable filter fluorometer, the hematofluorometer--which utilizes "front face" optics, internal standards, and digital computation capabilities--has been specifically designed for the rapid (approximately 5 s) assay of erythrocyte zinc protoporphyrin in unprocessed blood. A small (unmeasured) drop of blood, such as is obtained from a finger puncture, is placed on disposable cover slip and inserted in the sample holder of the instrument. The operator pushes the holder of the instrument. The operator pushes the holder, which also holds the permanent blank and standard, into the instrument. Zinc protoporphyrin concentration is automatically and instantaneously computed and the value is displayed on a digital readout as micrograms of zinc protoporphyrin per deciliter of blood for some standard hematocrit. No calibration adjustment need be performed by the operator. The hematofluorometer is designed for "field" use as well as for laboratory assays.