The biological reaction in the Tetrahymena population-growth impairment assay of exposure to chosen aliphatic aldehydes was investigated. A low Kow-dependent QSAR was also developed. Using the outcome of the study as guidelines, six derivatives were chosen for more assessment in the Tetrahymena mortality assay. The findings were contrasted for these two endpoints, together with the action of similar aldehydes measured in three other experimental systems. Each system was chosen to stand for a different protocol.