This study was undertaken to assess the effect of a known cytotoxic substance on the permeability of the organelle membranes of cultured cells. BHK-2-1 (C-13) cells were grown as monolayers on glass cover-slips and exposed for 1 h to 1 mg/ml and 2 mg/ml phenol. The cover slips were recovered and incubated for the demonstration of acid phosphatase and succinate dehydrogenase, marker enzymes for lysosomes and mitochondria respectively. The density of final reaction product after various incubation times was measured using a scanning integrating microdensitometer. At a concentration of 1 mg/ml, phenol causes labilization of the lysosomes and consequent loss of enzyme, which is further enhanced at 2 mg/ml. The mitochondrial membranes also showed a marked increase in permeability after exposure to 1 mg/ml phenol, but to a lesser extent than that of lysosomes. The results are discussed in the context of cell damage and DNA synthesis.