The memory cytotoxic T lymphocyte (CTL) response to human parainfluenza type 1 virus (hPIV-1), a prominent cause of respiratory infection in young children, has been analysed for a panel of healthy adults. The CTL response to the parainfluenza viruses has not been investigated previously. Precursor CTL (CTLp) with activity against hPIV-1-infected Epstein-Barr virus (EBV)-transformed B lymphoblastoid target cells were found at a relatively high precursor frequency (approximately 1/2500-1/4700 CD8+ and CD4+ subsets respectively) in peripheral blood. Both CD4+ and CD8+ CTLp were detected by the analysis of individual microcultures set up under limiting dilution conditions from freshly isolated blood, the phenotype of the responder cell from individual wells being determined by flow cytometry. Further characterization of the CTL response demonstrated MHC restriction by the HLA-A2 glycoprotein in 3/4 HLA-A2+ donors. The presence of effective, hPIV-1-directed T cell memory may explain, in part, the protection observed in the adult population.