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Theophylline For in vitro experiments, aminophylline (theophylline ethylene-diamine [Sigma Chemical Co.] or ethylenediamine [Wako Chemicals]) was diluted in complete medium to give the final molar concentrations as indicated. In the preincubation studies, effector cells were incubated with 10_3M aminophylline for 30, 60, 120, or 180 minutes and then washed extensively before the assay. For in vivo experiments, one sustained-release theophylline tablets (Theo-Dur; 200 mg and 100 mg) was used. Administration of Theophylline in Vivo The theophylline tablets, 400 mg or 500 mg per day (bid), were administered orally to five healthy normal volunteers for seven days. All of them were men, aged 25 to 35 years old. Informed consent was obtained in each case. Peripheral blood samples were collected before administration and 4 h after final administration on the final day of administration for the investigation of NK cell activity in the presence or absence of rhIL-2 (100 units/ml), serum theophylline concentration (by FPIA using unit-close kit; Dinabot Diagnostic Co), and lymphocyte subsets using monoclonal antibodies (OKT3, OKT4, OKT8, OKMI, Leu7, and Leulla; flow cytometric analysis). The NK cell activity was also assessed seven days after cessation of administration. Stastical Analysis Students t-test was used. All data were expressed as the mean±SD.