Background. Parathyroid tissue expresses the T-lymphocyte antigens CD3 and CD4, and parathyroid CD3 has earlier been proposed to interact in the regulation of parathyroid hormone (PTH) release. Methods. Anti-Leu3a, a monoclonal antibody recognizing CD4, was used to stain parathyroid tissue immunohistochemically, to influence PTH secretion from enzymatically dispersed parathyroid cells, and to immunoprecipitate parathyroid CD4. Northern blot and polymerase chain reaction were used to clarify the similarity between parathyroid and lymphocytic CD4. Serum PTH level was measured with an immunoradiometric assay in healthy control subjects and individuals with human immunodeficiency virus type 1. Results. The parenchyma of normal and abnormal parathyroid tissue displayed strikingly variable CD4 expression. Immunoprecipitation showed a 56 kd molecule, and Northern blot and polymerase chain reaction confirmed the similarity with lymphocyte CD4. Anti-Leu3a inhibited preferentially low calcium-stimulated secretion of PTH from dispersed parathyroid cells, without discernible influences on the cytoplasmic calcium concentration of these cells. Individuals with human immunodeficiency virus type 1 displayed significantly lower serum PTH levels than healthy control subjects. Conclusions: The results suggest that the human parathyroid chief cell expresses a CD4 moiety, which seems to interact in the PTH release in vitro and in vivo and which seems to use another second messenger system than the structurally similar T-cell equivalent.