Lead is a significant environmental pollutant in Portugal, mainly as a consequence of its use as an antidetonant in gasoline. Due to the vast annual gas consumption, evidence of lead emission has been found in the air, soil, plants, and water. Concentrations of lead in Lisbon waste waters average 0.15 mg/L, with occasional levels reaching 1.5 mg/L. The biodegradability of these waste waters is affected by lead. To understand the interaction between lead toxicity and protein concentrations, a study was initiated using selected strains of Proteus mirabilis and Klebsiella pneumoniae and synthetic effluents. The toxicity of lead to bacteria was noted by an increase in the cell's metabolic activity, and a greater induction of ATP and RNA contents. However, bacterial replication was evidently uninhibited by the lead content in the test medium. While high peptone content in the growth medium caused an increase in the cell's biosynthesis, increased lead concentrations in the growth medium resulted in a substantial loss of metabolic product. This complex interaction was apparently responsible for lead's toxicity to bacteria.