Current data indicate that MS2 sample stability over time in some waters was more variable than originally reported. This study examines different groundwaters and ultraviolet (UV) absorbers to see if there was an interaction that led to loss of MS2 infectivity. Samples of groundwaters from three different aquifers were collected for the study. Three UV absorbers (lignin sulfonate (LSA), coffee and humic acid) were used to adjust UVT to 75%. MS2 was added to each sample and phage concentrations were measured over a 50-day period to examine stability. Preparation of MS2 stocks and enumerative MS2 assay are discussed. Includes 8 references, figures.