Neutron activation analysis has for many years been considered to be a method that relatively is free from matrix interference. While this may be true for the majority of samples such as liquid, biological, air filters and geological aluminum-silicate specimens, there are many other types of samples that contains high Z materials in relatively high abundance that do attenuate low and medium energy gamma-rays. Samples such as heavy metal contaminated soils, ore concentrates and meteorites are prime examples. One can minimize the gamma-ray attenuation phenomena by using small masses of sample. This is not possible when using field environmental samples that are more than often not homogenous, and more sample mass is needed. Using a program called SELFABS, we have developed a very careful evaluation of the self-attenuation factors for a several of low and high Z reference materials. Special attention was given to the effect of the sample mass. Our conclusions indicate that while a great effort is given to various QA/QC procedures little literature information is ever published on the need or use for the calculation of these self-attenuation factors.