Immunoglobulin (Ig) “coating” on different in vivo growing mouse tumors was investigated using several approaches. Radioiodine‐labelled purified protein A from Staphylococcus aureus, a specific ragent for Fc of IgG, and/or iodinated pruified antibodies against mouse immunoglobulins, were fixed by various in vivo‐grown mouse tumor cells, but not by the corresponding in vitro‐cultivated tumor cells. Removal of host macrophages from the in vivo tumor‐cell preparations did not affect the fixation of anti‐mouse Ig reagents by the tumor cells. After an initial lag period in vivo, the intensity of the Ig coating of tumor cells increased with time after tumor inoculation. Conversely, the detectable coating decreased rapidly as a function of time after ascites tumor cells were explanted in vitro at 37°C. Iodoacetamide did partially block this 37°C in vitro‐induced “uncoating process”. Surfacebound Ig could also be released from in vivo‐coated tumor cells treated at low pH in vitro. Analysis of the behavior of tumor‐bound Ig indicated a composite pattern. At the membrane level, uncoating was best shown with in vitro incubation favorable to cellular metabolism; however IgG was detected equally well in the supernatants of the cells, whether they were incubated at 37° C or at 4° C, and so for has failed to display antibody activity towards uncoated tumor cells. This was in contrast to the case of IgG released from ascites tumor cells cultured at low pH in vitro: such eluted Ig, when neutralized, could rebind to the “same” uncoated tumor cells. Under favorable metabolic conditions, the fate of tumor‐bound Ig might be one of internalization and/or of degradation.