The toxic effects caused by the chemicals contained in wastewaters generated by laboratories involved in raw milk analyses were assessed using batch assays. These assays were carried out separately with methanogenic, ammonium-oxidizing, nitrite-oxidizing and denitrifying bacteria. Since sodium azide is one of the main components of the chemical mixture present in these streams, a set of assays was carried out with the whole chemical mixture, and another one was performed only with azide as the sole toxicant. The concentrations of all chemicals in the raw wastewaters (100%) were the fundamental references used to assess the relative concentrations corresponding to a decrease of 50% in bacterial activity (IC50). The results obtained showed that nitrite-oxidizing bacteria were the most sensitive microorganisms, with IC50 relative concentrations around 0.04%. The values obtained for the other groups were: 20, 20 and 33% for methanogenic, ammonium-oxidizing and denitrifying bacteria, respectively.