At the Palo Alto Reclamation Plant 0.044 cu m/s (1 mgd) of secondary effluent is reclaimed through a series of wastewater treatment processes. Mutagenic activity was consistently found to be present in the secondary treated municipal wastewater influent to the Reclamation Facility. This activity was not reduced significantly by high lime treatment, air stripping, recarbonation, or ozonation, even though these processes did remove a portion of the overall organic content of the wastewaters and many of the volatile organic compounds. Activated-carbon adsorption was effective in removing mutagenic activity to such a degree that mutagenic activity could not be found in water used for injection or that taken from monitoring wells. Chlorination resulted in an increase in mutagenic activity. A laboratory study demonstrated that this increase in activity would not result if chlorine dioxide rather than chlorine were used for disinfection. Chlorination resulted in the production of a significant concentration of non-purgeable, but otherwise unidentified, chlorinated organic compounds, as well as trihalomethanes. Formation or removal of such compounds did not seem to correlate with changes in mutagenic activity. Air stripping by the fountain-spray system was most effective in removing volatile organic compounds, while activated carbon was responsible for removals obtained for most other organic materials.