Toxicity Bioassay Validation as a Support for Monitoring Effluent Discharge -The present work aimed evaluating the impact of precipitation with sodium hydroxide and sodium sulfide, that clearly reduce the chemical contaminants of the effluent, leading to toxicity reduction. Metal analysis by ICP/AES (Inductively Coupled Plasma/Atomic Emission Spectroscopy), physicochemical analysis (pH) and toxicological analysis (by measuring the production of micronuclei in Allium cepa) were performed in the effluent studied before and after the precipitation. The analysis before treatment showed the extreme acidity of the effluent (pH<1,0), metals above the acceptable level for discharging according to the standards (especially zinc and iron) and significant mutagenicity. After precipitation with sodium sulfide, there was enough reduction in metals concentration to allow the effluent discharging within the limits specified by Conama Resolutions 357/2005 and 430/2011, and with both precipitants the discharging was possible according to the limits specified by 8468/76 Decree (São Paulo State/Brazil) (for the elements studied). There was also significant reduction in mutagenicity after treatment. The use of mutagenicity bioassay with A. cepa as a supplementary protocol to the ones already recommended for effluent analysis is emphasized in the present work.