In an attempt to detect and characterize immunoglobulins fixed in vivo onto tumor cells, a low pH buffer was used to dissociate the tumor cells from the bound immunoglobulins. IgG2 could be eluted from intact cells and from a membrane‐rich‐sub‐cellular fraction originating from primary muose banzo (a)pyrene‐indueced tumors and from spontaneous mammary tumors, indicating the association of this immunoglobulin with the surfac of the tumor cells. Significantly lower amounts of IgG2 were present in eluates of membrane‐rich fractioins originating from transplantable benzo(a) pyrene induced sarcomas. IgG1 was not detecetd in most of the tumor eluates. Membrane‐rich sub‐celluar fraction originating from normal mouse muscle did not contain any detectable immunoglobulin. On the basis of a semi‐quantitative estimation of IgG2 and IgGi in the serum of normal and tumor‐bearing mice, it was concluded that IgG2 is preferentially bound to tumor cells and is not a random contaminant of the cells or of the membrane‐rcih sub‐cellular fractions.