Intravenous administration of RA-2 transplantable rhabdomyosarcoma cells produces development of metastatic tumour cell colonies in the lungs of host rats. After administration of 2 X 10(3), 5 X 10(3) or 10 X 10(3) RA-2 cells, different patterns of dependence of the number of metastatic clones in the lungs on injected cell number have been observed. The numbers of tumour cell colonies were shown to be highest in immature (1-month-old) and old (15-month-old) female rats compared to middle-age groups (3- and 12-month-old). A similar age-related pattern was observed in male rats. Analysis of the results obtained has revealed positive correlation between tumour cell colony numbers and both somatomedin activity and proliferation rate of neoplastic tissue.