We examined the site-specific implantation of cancer cells in peritoneal tissues after an i.p. inoculation of 10(5) P388 leukemia cells. Twenty-four h after the inoculation, the number of viable cancer cells infiltrating into specific tissue sites of the peritoneum was estimated by an i.p. transfer method. A descending order of tissue implantation with cancer cells was established as omentum > gonadal fat > mesenterium > posterior abdominal wall > stomach, liver, intestine, anterior abdominal wall, and lung. A significant correlation was established between the logarithm of the number of infiltrating cancer cells and the logarithm of the number of milky spots. Next, the omentum was examined microscopically after i.p. inoculation with P388 leukemia cells labeled with bromodeoxyuridine or B-16 PC melanoma, which were differentiated from the other cells by an immunocytological method using anti-bromodeoxyuridine antibody or by the melanin of the B-16 PC melanoma cells. These cancer cells were found microscopically to be infiltrating only the milky spots, whereas none were seen at the other sites. These results suggest that cancer cells seeded i.p. specifically infiltrate the milky spots in the early stage of peritoneal metastases.