A type I interferon resistant variant (beta-MIP101) of the poorly differentiated human colon cancer cell MIP101 has a more aggressive phenotype in vivo in the nude mouse. Subcutaneous tumours grew at twice the rate of MIP101, but with similar morphology. beta-MIP101 also produced liver metastases at a higher frequency. beta-MIP101 tumours were diploid while MIP101 tumours were aneuploid. Both cell lines had doubling times of approximately 25 h in vitro.