Tobacco industry wastewater contains some toxic contaminants which inhibit the microbial consortium in biological treatment plants. Investigations carried out showed that the most important sources of these toxic contaminants are nicotine, flavoring chemicals containing glycogen and alcohol, absorbable organic halogens (AOX), and pesticides from tobacco leaves. Shock loadings of these toxic contaminants inhibit the bacterial activity during biological treatment. Input of wastewater containing such toxic substances causes decreases in bacteria numbers and reduce the treatment efficiency in the treatment plant. Viable numbers of floc forming, total and faecal coliform bacteria which are living in a biological treatment unit (i.e. Biopac) were monitored. Conventional short-term bioassays and enrichment toxicity tests were performed to determine potential toxicity. If the ratio of bacteria numbers growing in the enrichment medium compared to control is between 0.8 and 1.2, it may be assumed that toxic substances are absent. If this ratio exceeds 1.2, it may be assumed that growth stimulating substances are present. If this ratio is below 0.8, this indicates the presence of toxic substances. The results of enrichment toxicity studies showed that enough nutrients can stimulate the bacterial growth, or known/unknown sources of toxicity can be inhibit the microbial growth. Furthermore, results of chemical coagulation/flocculation test experiments showed that chemical treatment is effective at reducing toxicity and is proposed for biological treatment.