A 34,000-dalton peptide growth factor that we originally identified in human placental trophoblasts and in certain carcinomas was shown to be expressed in several normal human tissues. A highly specific antibody to the trophoblast-derived growth factor was used in an immunoperoxidase staining technique to identify the immunoreactive peptide in tissue sections. Immunoreactivity was seen in the adrenal cortex, Leydig cells of the testes, and follicular cells of the thyroid. In addition, strong staining was seen in the ducts and terminal ductules of the pancreas, in glandular epithelium of the prostate, in the chief cells of the stomach, and in the columnar epithelium of the trachea and bronchus of the lung. Certain tissues were negative for the peptide, including the adrenal medulla, liver, esophagus, small intestine, colon, bladder, lymph node, spleen, bone marrow, and thymus. Thus, the expression of the peptide depends on cell lineage and the state of differentiation; tissues of hematopoietic lineage are devoid of the 34,000-dalton peptide, whereas some of the major hormone-secreting tissues that are under pituitary control show the highest immunoreactivity.