Co-inoculation experiments in vivo. LM38-HP and LM38-D2 cells were cultivated together before subcutaneous injection, as described in the Materials and methods section. The combined tumors showed a higher incidence and number of spontaneous lung metastases than LM38-HP or LM38-D2 tumors. The results of two separate experiments are presented together. *P < 0.05 compared with LM38-HP and LM38-D2. Inset: a representative subcutaneous combined tumor stained for α-smooth muscle actin. Although myoepithelial cells surround epithelial cell nests, the characteristic papillary organization of M38 and LM38-LP tumors is not found.