Ehrlich ascites tumor cells were ectopically transplanted in femoral muscles of tumor-free Swiss and BALB/c mice with the same modality used for i.p. serial transplantations of the ascitic form. A solid tumor developed (100% takes as i.p. grafts) locally invading surrounding tissues and leading to death within 30-40 days (12-14 days in ascitic form). These animals were killed when showing signs of debilitation by tumor growth (1 mo.). The recipients' own thoracic and abdominal organs (lung, liver, spleen, and kidney plus peritoneal fluid) as well as the solid tumor were removed to obtain imprints and smears fixed and stained for cytology (May Grünwald Giemsa). Tumor-free mice were used as a control and i.p. transplanted mice were sacrificed on day 8. Disseminated tumor cells were seen in recipient organ imprints and peritoneal fluid smears scattered among local normal cells. Host defense cells with prevalence of neutrophils were observed infiltrating the solid tumor or adjacent to disseminated tumor cells. According to previous findings, organ imprints of i.p. transplanted mice showed disseminated tumor cells and host defense cells. Surprisingly, in liver imprints of ectopically transplanted BALB/c mice, numerous megakaryocytes were detected. This tumor and host organ imprint assay offers the possibility to monitor in vivo the phenomenon of metastatic tumor spread.