The article deals with the issue of determining the suitability index (SI) of radionuclidecontaminated (RN) agricultural and fishing products as an indicator of safety from the combined action on the human body of RN 137Cs and 90Sr. A new approach to calculating SI is presented. It combines the content of these RN in the product and the value of permissible levels of their activity. The approach is based on the assumption of the dependence of PP on the values of the activity multiplicity parameter. The parameters are determined in units of the minimum measured specific activity (MMSA) of the spectrometer for each of the RN. The influence of stock coefficients on the PP as a compensator of errors in the calculation of MMSA values is investigated. The stock ratio is defined as a function of the multiplicity parameter. The calculation of the values of the stock coefficient was performed using the model for describing the distribution of the relative error of activity measurement by the multiplicity parameter. The normalized Gaussian function was used. The argument of the latter one is represented by two parameters. One is the activity multiplicity parameter, the other one regulates the peak width. The parameters of the Gaussian function are determined by the method of minimizing the squares of deviations. At such values, the exponent optimally describes the experiment result. A universal equation for SI calculation has been obtained. The equation combines two separate ratios, according to which the SI relative to the combined effect of RN 137Cs and 90Sr is usually calculated when analyzing the radiation quality of food raw materials and food. The results of calculations of SI, stock coefficient and necessary parameters for this purpose according to the measurement of RN 137Cs and 90Sr activity in mushrooms of forest origin are presented. The possibility of preliminary estimation of technological parameters of test is shown. The method of reducing the study time by obtaining data using concentrated and non-concentrated samples is substantiated. The efficiency of the SI calculation algorithm in the case of multiplicity parameters, the value of which is close to the measurement low limit, is proved. The presented ratios provide an additional guarantee in the correctness of the decision on the suitability of the product