The in vitro effect of leucocyte α‐interferon on human myeloma cells in a semisolid agar culture system

73 bone marrow samples from a total of 50 patients with multiple myeloma (MM) were tested for interferon (IFN) sensitivity in the human tumour colony assay (HTCA). 16 evaluable samples were obtained from untreated patients, 10 from patients during melphalan and prednisone treatment and 6 from patients on IFN treatment or after withdrawal of IFN. The sensitivity to IFN was individually distributed between the bone marrow samples from patients of all 3 groups and a tendency towards IFN resistance could be found in serial assays from patients during MP-treatment and after IFN withdrawal. Stimulation of growth was found in 31% of the cultures, usually at lower doses of IFN (10-100 U/ml) while higher doses (400-4000 U/ml) inhibited growth in 75% of the assays.