Cross-linking of the Ag receptors on lymphocytes initiates activation of the receptor-coupled tyrosine kinases. HS1 is one of the substrates of these kinases and has been shown to transduce the signals for both clonal expansion and deletion in lymphoid cells. To gain further insight into the mechanism of action of HS1, we have tried to identify a protein that interacts with HS1 by yeast two-hybrid screening. The isolated cDNA, designated HAX-1, encodes a novel 35-kDa protein. The HAX-1 gene is expressed ubiquitously among tissues, and its protein is localized mainly in mitochondria, but also in endoplasmic reticulum and nuclear envelope in the cell. HS1/HAX-1 association is confirmed by coimmunoprecipitation of these proteins in the lysates of B lymphoma cells and COS-7 cells transfected with the corresponding cDNA expression vectors. Colocalization of these proteins in the cell is evident under confocal laser scanning microscope. Deletion mutant analysis of these proteins reveals that the association is mediated by the amino terminal region of HS1 and the carboxyl-terminal half of HAX-1. The potential role of the HAX-1/HS1 complex is discussed.