With a view to the determination of trace amounts of arsenic in microbiologically produced protein feeds, the authors compared the results yielded by the photometric method using silver diethyl dithiocarbamate ( AgDDTC ) with those obtained by means of the atomic absorption spectrometry(AAS)-hydride technique. Prior to analysis, the samples were dry-ashed after addition of Mg/Mg(NO3)2 or digested with nitric acid. Further comparative results were obtained by neutron-activation analysis. These methods were used to analyze novel protein feeds produced from yeast and bacterial biomasses, as well as some fermentation raw materials, for arsenic. A comparison of the values found with literature data relative to the arsenic contents of conventional protein feeds and foods shows that the novel microbiologically produced protein feeds are absolutely able to compete with protein feeds and foods currently used.