A culture medium promoting the in vitro nodulation of Alnus crispa var. mollis Fern. was established. Under well-defined growth conditions, a minimum inoculum of about 1 μg of fresh nodule per plant was sufficient to promote good nodulation of the host plant. Using the suspension–dilution technique combined with inoculation tests, it was possible to eliminate progressively endogenous contaminants and to obtain axenic nodules. These nodules are now being used in attempting to isolate the causal organism of the nodule.