Sera from 151 patients with a variety of cancers were screened for antibody-like activity against lipoproteins. Eighteen % of the sera exhibited activity against autologous and homologous high-density lipoproteins and 3% exhibited activity autologous and homologous low-density lipoprotiens. Antibody-like binding was proven by its restriction to the Fab fragment of IgG. The reactive part of the lipoprotein molecule was shown to be the appoprotein. Quantiation of the serum lipoproteins indicated that thehigh-density lipoprotien concentration in the sera of cancer patients was significantly lower (psmaller than 0.01) when antibody was present. These observation suggest that autoimmune mechanisms may be responsible for the decreased high-density lipoprotein serum levels in some patients with cancer.