Inactivation of E. coli was investigated via standard plate culture assay and a molecular based assay that measured dimmer formation. Experiments were performed under radiation from conventional low and medium pressure ultraviolet (UV) sources as well as wavelength bands around 228 and 289 nm. Inactivation of E. coli was similar under LP and MP sources and the relative number of dimers formed per UV dose was similar. Inactivation was much slower under the other wavelength bands tested but 228nm was more effective than 289 nm radiation. These results support the accepted action spectrum for E. coli and suggest that for inactivation of this bacteria there is no significant difference between LP and MP sources. Includes 6 references, table, figures.