SUMMARYLeukocytes, subjected once to interferon (IFN) induction by HVJ (Sendai virus), were studied for their capability to produce IFN after a second similar stimulus. Substantial amounts of IFN (about 30000 IU/ml) were recovered. Experiments using cycloheximide or actinomycin D and kinetic studies showed that this IFN originated mainly in IFN which resided within the cell as a result of the first induction and was released after the second stimulation. Increasing amounts of HVJ used for the second stimulus resulted in proportionally increased yields of IFN, reaching a plateau at the same dose of HVJ (1000 HAU/ml) as that which gave optimal yields after the first stimulation. Evidence is presented that the capacity of HVJ to trigger the production of a second IFN harvest is closely associated with its infectivity.Interferon (IFN)-ct derived from leukocytes of healthy volunteers is produced at the Osaka Blood Center using the method of Mogensen & Cantell (1977). Despite the advantages of this system, leukocytes are only available in limited quantity. Therefore, we investigated the possibility of re-use of the leukocytes for IFN production, and found that a second yield of about 30000 International Units (IU)/ml of IFN could be obtained.The pooled buffy coat layers from healthy donors were centrifuged at 3000 r.p.m, for 9 min and the concentrated buffy coats were collected. They were haemolysed with cold 0.83 ~ NH4C1 (adjusted to neutral pH with NaHCO3) and then centrifuged at 800 r.p.m, for 10 min. The cells were again treated with NH4C1, then centrifuged as above and washed with Ham's F-12 medium (Nissui Seiyaku Co. Ltd., Japan). The purified leukocytes were suspended in the same medium containing 2~ human serum at a concentration of 1 x 10 v cells/ml and incubated at 37 °C for 2 h. HVJ (Sendai virus) was added (final concentration 100 HAU/ml) and the leukocytes were incubated for an additional 14 to 16 h. At the end of this period the supernatant fluid was harvested and the cells were collected by centrifugation at 4200 r.p.m, for 25 min. The cells were resuspended in Ham's F-12 medium for reculturing. The recultured cells were handled in a similar manner to the first culture with modifications of cell concentration and HVJ concentration (see below). After incubation, the supernatant fluid was harvested.Titres of IFN were determined on human amnion (FL) cells by a cytopathic inhibition microassay method with vesicular stomatitis or Sindbis virus as a challenge virus. All titres were calibrated against the international IFN-ct reference and are expressed as IU/ml. The human IFN-ct antiserum (NIH G026-502-568) was used to neutralize IFN activity. IFN samples (approx. 200 IU/ml) were serially diluted (twofold) and each dilution was mixed with an equal volume of antiserum. After incubation at 37 °C for 60 rain, the residual IFN activities were assayed as described above.HVJ (provided by Dr T. Kishida, Cantell strain) was inoculated into embryonated eggs. Infected chorioallantoic fluids were purified by differenti...