Abstractα‐Aminoisobutyric acid transport has been demonstrated in cultured human epidermal keratinocytes as well as in the transformed state of these cells. The concentrative uptake is sodium‐dependent and may be ascribed to one Michaelis‐Menten component whose maximal velocity is 6.9 nmol/min × 106cells, with an apparent affinity of 3.8 mM. These parameters may be modified, depending upon the nature of malignant transformation. In SVK14 SV40‐virus‐transformed cells, there is no change of affinity but the maximal velocity is 1.5 fold less than in normal cells. In the spontaneous epidermoid carcinoma‐derived A431 line, this phenomenon is inverted; the maximal velocity is unmodified but the system affinity is 2.2‐fold higher than in normal cells. The unresponsiveness of this transport system to insulin and epidermal growth factor (EGF) shows that it behaves differently from those of many o