The auxin-binding protein, ABP1, has been postulated to mediate auxin-induced cellular changes associated with cell expansion. This protein contains the endoplasmic reticulum (ER) retention signal, the tetrapeptide KDEL, at its carboxy terminus, consistent with previous subcellular fractionation data that indicated an endoplasmic reticulum location for ABP1. Our work uses electron microscopic immunocytochemistry to identify the subcellular localization of ABP1. Using maize coleoptile tissue and a black mexican sweet (BMS) maize cell line, we have found that ABP1 is located in the ER as expected, but also on or closely associated with the plasma membrane and within the cell wall. Labeling of the Golgi apparatus suggests that the transport of ABP1 to the cell wall occurs via the secretory system. Inhibition of secretion of an ABP homolog into the medium of BMS cell cultures by brefeldin A, a drug that specifically blocks secretion, is consistent with this secretion pathway. The secreted protein was recognized by an antiKDEL peptide antibody strongly supporting the interpretation that movement of this protein out of the ER does not involve loss of the carboxy-terminal signal. Cells starved for 2, 4-D for 72 hours retained less ABP in the cell and secreted more of it into the medium. The significance of our observations is twofold. We have identified a KDEL-containing protein that specifically escapes the ER retention system, and we provide an explanation for the apparent discrepancy that most of the ABP is located in the ER whereas ABP and auxin act at the plasma membrane.