Iscador activated (in vivo and in vitro) splenocytes were found to inhibit metastatic tumour growth in C57BL/6 mice. In order to check whether NK cells are involved in the antimetastatic activity of Iscador activated splenocytes ,animals were depleted of NK cells using antiasialo GMI antibodies. When spleen cells activated in vivo with Iscador were injected into animals pretreated with Antiasialo GM I antibodies, there was an average of 44.6 tumour nodules on 21st day indicating that stimulation of NK cell activity produced by the Iscador compensate for the NK cell depletion by Antiasialo GM I antibody. Animals treated with Iscador activated splenocytes showed an average survival period of 68 days whereas that of control tumour bearing animals treated with Ab the average survival was 19.3 days. The lung collagen hydroxyproline content, serum sialic acid levels, markers of metastasis were also significantly (P<0.001) lowered in these animals compared to those of the untreated tumour bearing animals. gamma-glutamyl transpeptidase a marker of neoplastic growth, was also significantly reduced (P<0.001) in animals treated with activated splenocytes.