We studied the growth characteristics and hypoxic fractions of DLD-2 human colon tumours xenografted into male nude mice either in the unperturbed state or after i.p. injection (q.i.d. x 7) of basic fibroblast growth factor (0.25 mg kg-1) or suramin (50 mg kg-1). Hypoxic fractions were measured by clonogenic excision assay 1 day after administration b FGF or suramin was stopped. As compared to controls, the growth of tumours in b FGF treated mice was increased by a factor of 1.5 as indicated by the relative volumes of tumours on the day of excision. Similarly, suramin decreased the growth of DLD-2 tumours by a factor of 1.6. The percentage of hypoxic cells in control neoplasms was 42.9% (95% confidence limits 34.2-52.1%). In mice that received basic fibroblast growth factor injections, hypoxic fractions decreased to 19.1% (95% confidence limits 13.5-26.9%). In contrast, in mice treated with suramin, the percentage of hypoxic cells increased to 74.0% (95% confidence limits 65.3-83.9%). These data indicate that the biology of solid tumours can be significantly modified by alteration of growth factor status.