Tissue sections and cell suspensions from ten malignant tumors were tested for Fc receptors using sheep erythrocytes sensitized by rabbit IgG antibodies. Surface bound IgG on cells from the same tumors were estimated using an antiglobulin consumption test with 125I labelled human IgG as reference. The amount of IgG present per 10(6) cells varied from less than 100 ng to approximately 600 ng. When these amounts of IgG were plotted against the Fc receptor activity of corresponding tumors, seven of the tumors were distributed along a line showing an inverse linear relationship; i.e. tumors with large amounts of IgG on their cell surfaces had the lowest Fc reactivity and vice versa. Cells from three of the tumors had lower amounts of IgG on their surface than expected from this relationship. However, the lack of correlation could be explained by the focal distribution of the Fc positive tissue within non-reactive tissue. The cells from these areas presumably carry less IgG on their surface and thereby reduce the quantity of IgG calculated per 10(6) cells. Prolonged washing of tumor sections resulted in stronger Fc receptor activity, and correspondingly washed cells had lower amount of IgG on their surface. Presumably the Fc receptor can bind IgG in vivo.