In this paper we present evidence that antibodies unrelated to the tumor cell can comprise part of the in vivo Ig surface coat of cells derived from the non-lymphoid murine tumor, TA3/St. This was shown by incubating TA3 cells originating from other OA or BSA preimmunized mice with radioiodinated ¹²⁵I-OA and ¹³¹I BSA. Radioiodine-labeled ¹²⁵I-OA specifically fixed to cells derived from TA3/St tumors originating from OA-preimmunized mice. On the other hand ¹³¹I-BSA was specifically fixed by cell populations from BSA-preimmunized mice. Incubation of these cells in vitro at 37²C abolished the specific binding and antibody could subsequently be detected in the tissue culture medium.

Radioiodine labeled purified soluble antibody-antigen complexes could also be bound to cells derived from freshly harvested TA3/St tumors but not to their in vitro propagated counterparts. Removal of phagocytic or adherent cells from these cell populations decreased the binding of the complexed antibody on freshly harvested TA3/St populations, but did not eliminate it. Inhibition of complexed antibody binding was obtained when TA3/St cells (an H-2^a tumor) were pre-incubated with anti H-2^a antiserum. Propagation of the tumor in an F₁ hybrid (A × C57BL) in which host cells could be distinguished from tumor cells by using an anti H-2^b antiserum showed that binding of the immune complex was mostly limited to host cells infiltrating into the tumor population.