The effects of all trans retinoic acid and hyperthermia were studied in the human colon adenocarcinoma cell line HT29. Cell cytotoxicity after exposure to ATRA or heat-shock, alone or in association, was evaluated by the MTT assay while cell surface and ultrastructure modifications and actin fibre assembly changes were investigated by electron microscopy and by the FITC-phalloidin method. Apoptosis was evaluated by flow cytofluorimetry and electron microscopy. Reverse transcriptase-polymerase chain reaction was employed to study mRNA expression of genes involved in apoptosis, differentiation and growth arrest. Joint treatments were more effective in reducing the vital cell yield, being this effect only partially due to apoptosis. A marked up-regulation of the cyclin-dependent kinase inhibitor p21WAF1/Cip1 expression, not followed by any differentiation process, was responsible for growth arrest. Modulation of Hsp-70 expression, involved in cell response to treatments, was considered. Our results demonstrate that cell treatment with ATRA followed by heat-shock may elicit useful effects to treat tumours, which are responsive to retinoids, as well as those malignant cells which may be constitutively thermotolerant.