Carboxyhemoglobin (HbCO) is formed when carbon monoxide is bound to hemoglobin. High levels of HbCO are known to reduce the amount of oxygen that can be carried to the tissues. This experimental study focuses on the influence of HbCO on the growth, blood flow, and radiation response of an experimental mouse tumor. The study was designed to mimic the clinical situation where heavy smokers are undergoing radiotherapy while having a high HbCO level. The tumor was a C3H mammary carcinoma grown in the feet of CDF1 mice. Chronic exposure to carbon monoxide, resulting in 10% HbCO, increased the tumor volume doubling time from 2.5 to 3.5 days (p less than 0.05). The acute exposure to carbon monoxide prior to and during irradiation significantly raised the radiation dose required to control the tumor locally. The TCD50 increased from 54 Gy in air breathing mice (HbCO 0-2%) to 57 Gy (HbCO 7-9%) and 61 Gy (HbCO 20-23%). This increase corresponded to an increase in the fraction of clonogenic hypoxic tumor cells from 0.12 in air breathing animals to 0.21 and 0.41, respectively. The tumor blood flow, determined by the 86RbCl extraction technique, decreased to 63% (n.s.) and 50% (p less than 0.05) for low and high HbCO levels, respectively.