Nine patients with mid-gut carcinoid tumours received leucocyte interferon (IFN) i.m. daily for 90 days. Six patients clearly ameliorated in symptoms typical of the carcinoid syndrome (flushing, diarrhoea, asthma) which correlated with reduced serum levels of tumour related polypeptides and urinary output of 5-hydroxyindole acetic acid (5-HIAA). Before IFN treatment, peripheral blood mononuclear leucocytes (PBLs) from carcinoid patients showed markedly deficient production of pH 2 labile IFN-alpha in response to Staphylococcus aureus Cowan I (SACoI) in vitro. In contrast, IFN-alpha responses to the inducers Sendai virus and beta-haemolytic streptococcus group G and IFN-gamma responses to Lens culinaris lectin and concanavalin A were normal. Also, basal and in vitro IFN enhanced natural killer (NK) cell activity and T cell mitogen-induced cell proliferation were similar in patients and controls. During 90 days of IFN therapy, SACoI-induced IFN responses became entirely undetectable. There were transient declines at 1 and 30 days in IFN responses to the other IFN inducers, of mitogen-induced lymphocyte proliferation and of basal NK activities. The increments of NK cell activities after in vitro IFN exposure were clearly decreased in IFN treated patients, suggesting in vivo activation of these cells. Thus, the results demonstrate one remarkable abnormality in carcinoid patients: a deficient IFN response to SACoI and a clear influence of IFN therapy on several parameters of the IFN-NK system.