The authors have reviewed the techniques developed for measuring the concentration of total arsenic and that of some specific arsenic species (As V, As III, aromatic and aliphatic arsenic derivatives) in biological materials mainly in urine. Whether the relative concentrations of these arsenical species found in human urine represent their true distribution in the urine secreted by the kidney (possible change in valence state in the bladder mainly at alkaline pH, breakdown of arsenical derivatives during urine pretreatment) remains to be investigated further. No technique is yet available for measuring specifically the organoarsenic derivative(s) excreted after ingestion of marine animals. No available system has been found that will digest tissues without changing the original valence states of arsenic. Therefore only total arsenic can be measured with accuracy in tissue.