The results given below show that, whilst it is not possible to transfer immunity to infections with Nematospiroides dubius passively with serum from immune mice to normal adult mice, young born to immune females are resistant to this infection. The immunity is dependent on an intake of immunoglobulin via the milk for a period longer than 24 h. The passive transfer of immunity from immune mothers to neonatal mice does not appear to be dependent on a specific class of immunoglobulins.