[en] A novel method is described by which nuclear activation analysis can be used to determine the chemical form in which certain elements occur. The radiochemical retention of molecular identity is constant for a given type of sample and can be easily determined. Application of this method for quantitative estimation of methylmercury in fish protein concentrates and in blood is described in detail and the results discussed. An alternative method is also described which, although less direct, is free from many of the common interferences, such as sodium and phosphorus. This method, also useful for blood analysis, is appropriate for use at remote sampling stations. (author)