The principal aim of this study was to determine whether the sodium butyrate, cell differentiation-inducing compound, induces identical morphological changes in two colorectal adenocarcinoma cell lines which exhibit the different changes in the alkaline phosphatase activity after treatment with this agent. Ultrastructural analysis showed that these two cell lines possessed different sensitivity to the presence of sodium butyrate. Particularly different changes were observed in the chromatin structure of the cell lines tested. Our study demonstrates that sodium butyrate initiates cell differentiation, modifies the cell components, but the characteristics and extent of this modification depends on the cell line used.