A tumour line inoculated in mice showed high affinity binding for lipoproteins in vitro. Studies in vivo demonstrated that the assimilation of human low density lipoprotein (LDL) by the tumour was very high. Both receptor and non-receptor mediated catabolism of the lipoprotein by the tumour increased as compared to other tissues known to be sites of lipoprotein catabolism (liver, spleen etc.). These findings suggest that lipoproteins may be useful markers for tumours as well as carriers for cytotoxic drugs to target tissues in vivo.