A murine mammary carcinoma, which had a high potential for metastasis to the lungs, was established in culture, and from the parent line several clonally derived variants were isolated, showing different characteristics including metastatic potential. C1, a high metastatic clone, and C2, a low one, were selected for further study. When tumour cells were injected s.c. the growth rates of the resulting tumours were higher when they developed from the parent line (P2) or C1 cells, than from C2 cells. The numbers of lung colonies seen following i.v. inoculation of tumour cells also varied, C2 cells yielding the lowest score. In vitro C1 cells were more efficient at aggregating platelets than C2, an effect reduced by the addition of heparin. In vivo heparin reduced the number of tumour cells arrested in the lungs after i.v. injection, and also the number lung colonies which subsequently became established. The number of metastases which developed following s.c. injection of tumour cells was also reduced by heparin.