Because of its apparent structural integrity, cement has been widely used in the United States as a binder to solidify Class B and C low-level radioactive waste (LLW). However, the resulting cement preparations are susceptible to failure due to the actions of stress and environment. An environmentally mediated process that could affect cement stability is the action of naturally occurring microorganisms. The U.S. Nuclear Regulatory Commission (NRC), recognizing this eventuality, stated in their Technical Position on Waste Form, Revision 1, that the effects of microbial action on waste form integrity must be addressed. This paper provides recent results from a program that examined the effects of microbially influenced degradation (MID) on cement-solidified LLW. Data are provided which were obtained during the development of an evaluation method using acid-producing bacteria. Results presented here are from work with one type of these bacteria, the sulfur-oxidizing Thiobacillus. Commercially prepared, cement-solidified, low-level radioactive waste form samples made from power reactor wastes were evaluated using a new biodegradation test developed for the NRC. Testing demonstrated that MID has the potential to severely compromise the structural integrity and nuclide retentiveness of ion-exchange resin and evaporator-bottoms wastes that have been solidified with cement. It was found that the waste form specimens physically deteriorated after 60 days of exposure to the thiobacilli. Also, the data show that significant amounts of Cs-137, Cs-134, Co-60, C-14, Tc-99, and Sr-90 contained in the waste forms were leached in the presence of Thiobacillus.