An immunoperoxidase method has been developed to detect parathyroid hormone-related protein (PTHrP) in histological specimens of tumors and of normal skin. A rabbit polyclonal antiserum against PTHrP-(1-16) was used that did not cross-react with PTH-(1-34) either under radioimmunoassay conditions or at the high antiserum concentrations used in neutralizing biologic activity. PTHrP antigen was detected in the keratinocyte layer of normal skin and in 100% of 34 samples of squamous cell cancers but in only one of six breast cancers, and none of 15 other adenocarcinomata. It was also detected in four of four samples of renal cortical carcinoma and two of two of melanoma, both of which can be associated with hypercalcemia, and three of three small cell carcinomata of the lung. Immunologic detection of PTHrP could be useful in the diagnosis of tumors of squamous cell origin, particularly in the cytological differentiation of lung cancers, where it may be of value in distinguishing between squamous cell and small cell carcinoma on the one hand and poorly differentiated adenocarcinoma on the other.