Two biopsies of intraductal and invasive lobular carcinoma of the breast and 11 biopsies of intraductal and invasive carcinoma of the breast were examined by automated micromensurative techniques for mean nuclear area of carcinoma cells in intraductal and in invasive compartments. The nuclei of invasive carcinoma cells tended to be smaller when they invaded stroma as single cells or as "thin strands" than when they invaded as part of large sheets of cells. Cells in direct apposition with the stroma usually had smaller nuclei than cells which were found centrally in large confluent sheets of cells. Intraduct carcinoma cells approached, in size, invasive cells which formed confluent sheets, rather than cancer cells which invaded as single cells. The nuclei of cells which invaded in small groups were usually darker than those of cells in confluent sheets, which were usually more vesicular.