The blocking effect of conditioned media from tumour cell cultures on the lysis of target cells by immune lymphocytes and macrophages.

Conditioned media prepared from tumour cell cultures of three mouse strains and man blocked the lytic reaction of target cells by allogeneic immune lymphocytes and macrophages. No effect was exerted on this reaction; when conditioned media prepared from non-tumour cell cultures were utilized. The lytic reaction was even enhanced in some cases. It is suggested that tumour cells cultured in vitro release into culture media substances which may inhibit the lysis of target cells by lymphocytes and macrophages.