AIMS: To save time and labour in mass screening, by detecting two marker proteins on one specimen using only one test. METHODS: alpha Fetoprotein and ferritin were chosen to demonstrate the principal of this system. The assay reagents were horseradish peroxidase (HRP)-labelled anti-alpha fetoprotein and HRP-labelled anti-ferritin antibodies. After the serum sample had been incubated with these reagents the substrate for HRP was added and the absorbance measured. An absorbance value below the cutoff point indicated that both parameters were within normal limits; a value above the cutoff point indicated that at least one of the two parameters was abnormally high. RESULTS: Fifty sera from healthy Japanese subjects were assayed by the simultaneous assay method. All samples gave absorbancy values below the cutoff point. Fifty serum samples from patients with high alpha fetoprotein concentrations (over 20 ng/ml) and 50 samples with high ferritin concentrations (over 200 ng/ml) were also assayed. The absorbancy values of all samples with high alpha fetoprotein concentrations, and all but one sample with high ferritin concentrations gave values above the cutoff point. CONCLUSIONS: Although this homogeneous enzyme assay method was applied to the combination of alpha fetoprotein and ferritin, it could be used in mass screening for any other combination of two markers.