The toxicity of trichloroethylene was tested in both glass enclosures and in polystyrene immunological plates, and resulted in insignificant correlation of EC₅₀ (evaluated as OD 750 nm). In enclosures, EC₅₀ evaluated as O₂ evolution and as pH increment was highly significant. The optimal time for the assay of TCE in enclosures was 48-72 h; and in plates ca 200 h. After a longer time (e.g. 300 h) TCE diffused from the plates and growth was revived. TCE in low concentrations (ca. 0.1-0.2 g/L) stimulated the growth of some tested organisms, both in plates and in enclosures. Toxicity occurred from ca. 0.35-0.6 g/L (EC₅₀). The testing procedure in enclosures was a suitable method for some volatile substances, giving rapid results.