Tumour cells were shown to secrete substances stimulating macrophage migration in vitro and also those depressing the cytopathogenic action of immune lymphocytes in vitro. Embryo fibroblasts (examined in paralled as nontumour cells) secreted as did tumour cells, substances stimulating the macrophage migration, but not substances capable of depressing the cytopathogenic action of immune lymphocytes. L-cells secreted substances inhibiting macrophage migration, but failed to secrete substances depressing the cytopathogenic action of immune lymphocytes.