Human tumors frequently present heat shock protein 70 (Hsp70) on their cell membranes, whereas corresponding normal tissues fail to do so. Therefore, an Hsp70 membrane-positive phenotype provided a tumor-specific marker. Moreover, membrane-bound Hsp70 provides a target structure for the cytolytic attack mediated by natural killer (NK) cells. Vitamin A derivatives 13-cis retinoic acid (13-RA) and all-trans retinoic acid (ATRA) and sodium-butyrate (SBU) are known for their redifferentiating capacity. Therefore, we asked the question whether loss in tumorigenicity might be associated with a reduced Hsp70 membrane expression. For our studies we used epithelial colon (CX /CX−) and thyroid (ML-1) cancer cells, with initially different Hsp70 cell surface expression pattern. After treatment up to 7 weeks with freshly prepared 13-RA, ATRA, and SBU at nonlethal concentrations of 10 μM, 1 μM, and 0.5 mM, respectively, growth morphology, Hsp70 levels, and sensitivity toward Hsp70-specific NK cells were compared with that of untreated tumor cells. Significant growth delay was determined in CX tumor cells after 6 weeks treatment with 13-RA. Concomitantly, growth morphology changed from spheroid cell clusters to monolayers. Despite a weak increase in cytosolic Hsp70, the percentage of Hsp70 membrane-positive cells dropped significantly after repeated treatments with 13-RA and ATRA in CX and ML-1 but not in CX− tumor cells. Similar results were observed with SBU. Functionally, the decrease in Hsp70 membrane-positive CX and ML-1 cells correlated with a reduced sensitivity to lysis mediated by NK cells. In summary, redifferentiating agents predominantly affected Hsp70 membrane-positive tumors. The decrease in Hsp70 membrane positivity correlated with a lower sensitivity to NK lysis, growth delay, and altered growth morphology.