Tumour cell proliferation shows a heterogeneous intratumour distribution. By comparison with the infiltrating component of breast cancers, the intraductal component has a significantly lower proliferation index. The cells at the periphery of infiltrating tumour strands have a higher proliferation activity than the cells in the core. A variable turn-over of basement membrane material is reported in infiltrating cancers. Increased amounts of type IV collagen are demonstrated in areas of periductal elastosis and of interstitial elastosis in breast cancer. Important parallels are found between metastatic tumour cells and the macrophages acting in the process of inflammation. We found evidence that displacements of tumour cells and macrophages are similar. Studies of vascularization in transplanted tumours cannot be extrapolated to man. A striking heterogeneity in the organization of vessels and in the expression of some markers is observed in human breast cancer.