The effect of azathioprine and inosiplex on the capacity of peripheral blood lymphocytes to form E active rosettes has been studied in vitro on 25 healthy subjects and 63 cancer patients (27 with pulmonary cancer and 36 with cancer of the digestive tract). Azathioprine significantly inhibited the capacity of T cells obtained from normal donors to form rosettes (P less than 0.01). Similar results were registered in patients suffering from pulmonary cancer (P less than 0.01) or from digestive neoplasia (P less than 0.05). Inosiplex, a substance with antiviral and immunomodulating properties, increased the percentage of the E active rosettes forming cells of 18 subjects with lung carcinoma and 26 patients with cancer of the gastrointestinal apparatus. If lymphocytes of both normal donors and cancer patients were preincubated simultaneously with azathioprine and inosiplex, the latter had an inhibitory effect on the azathioprine dependent rosette inhibition and restored the percentage of the E active rosette forming cells to control values.