Prostaglandin F2 alpha (PGF 2 alpha) has a bronchoconstrictive effect on the smooth muscles of the bronchial tree, while a prostaglandin of the E series (PGE) provokes bronchodilatation in bronchial asthma (BA). The PGE series can suppress cytotoxic T lymphocytes and it was expected that PGF2 alpha would have the opposite effect, since similarities exist between microvilli of circulating blood lymphocytes and bronchial smooth muscles. Pharmacological modulation of lymphocytes, using a mitogen-induced, cell-mediated cytotoxicity assay (MICC), showed that PGF2 alpha suppresses cytotoxic activity of lymphocytes in BA patients and normal controls. This observation provides evidence that the spasmogenic effect of inhaled PGF2 alpha on the smooth muscles of the airways does not correspond to expected increased cytotoxicity of the circulating blood lymphocytes in vitro.