Abstract In mammalian cells the pH gradient between the organelles, cytoplasm and extracellular space is strictly regulated. Maintenance of pH homeostasis is crucial for the normal function of the cell and its organelles. In solid tumours, cells often suffer from hypoxia, deprivation of nutrients and acidic extracellular milieu as a result of inadequate vascularisation. Cancer cells are also known to suffer from other pH abnormalities. Defective acidification of intracellular organelles as well as a reversed pH gradient across the plasma membrane have been detected in numerous tumour tissues and cells. Aberrant secretion of lysosomal hydrolases, loss of cell polarity and increased expression of tumour-specific proteins are common phenotypic changes of cancer cells. In this study, secretion of cathepsin D, a lysosomal aspartic hydrolase, was shown to result from the acidification defect of cancer cells. In normal cells cathepsin D is sorted in the Golgi complex by mannose-6-phosphate receptors and transported via endosomes to lysosomes. In breast and colorectal cancer cells having abnormally neutral endosomes receptors were shown to accumulate in endosomes resulting in the aberrant secretion of newly synthesised cathepsin D from the cells. Carcinoembryonic antigen (CEA) is an oncofetal protein widely used as a tumour follow-up marker. It is normally expressed at low levels and is localised at the apical surface of epithelial cells via a glycosyl phosphatidyl inositol (GPI) anchor. In cancer cells the expression of CEA is increased and the protein is found over the entire surface of cells. In this study, the tumour microenvironmental factors, hypoxia and abnormal pH homeostasis, were shown to increase the expression of carcinoembryonic antigen in cancer cells. In addition, the absence of acidic organelles was shown to induce mistargeting of CEA to the basolateral membrane in polarised cells. The abnormally neutral Golgi was found to interfere with the complex formation of carcinoembryonic antigen, a phenomenon recently associated with the apical sorting of other GPI-anchored proteins. Altogether these results emphasise the role of tumour-related factors – altered pH homeostasis and hypoxia – in the phenotypic changes of cancer cells.