Intradermal, interlesional and intravenous MER/BCG have been reported to possess immunostimulatory properties and combined with chemo-radiotherapy an anti-neoplastic effect. Due to local and systemic side effects of the methanol extraction residue of BCG therapy a new approach to oral administration was investigated. Seventeen patients with inoperable non-oat cell lung cancer were given oral MER for 30 days. Skin tests to 5 recall antigens and various concentrations of MER, lymphocyte stimulation by PHA were done before and repeated during therapy. The initial group of patients received a dose of 1.25 mg per day and when no side effects were detected the dose was gradually escalated in subsequent groups of patients up to 5 mg. Oral MER was well tolerated even at the higher dose with no clinical or laboratory side effects. No regression in tumor size was seen. In 6 of 17 patients the disease remained stationary for a mean of 6 months (Range 6-14 months). In the remainder, disease progressed after a mean of 15 weeks. Two patients had cutaneous PPD reactivity converted from negative to positive, in one it became negative, while the remaining patients maintained their original responsiveness. No major changes could be observed in the in vivo immune tests performed following the course of treatment. In view of the reported relative efficacy of oral BCG administration, and considering the very low toxicity with oral MER, further studies employing considerably higher doses of this nonviable vaccine are now justified.