We previously demonstrated that the specific component of rat urine designated as Fraction I (Fr.I), which has been known to enhance carcinogenesis in the rat urinary bladder, contains epidermal growth factor (EGF) and transferrin (TF). The present study was designed to determine whether EGF or TF is responsible for the tumor-enhancing effect of Fr.I. The heterotopically transplanted rat urinary bladder (HTB), which has been developed in our laboratory, was used for the study. Fr.I was prepared from normal rat urine by a method published previously. Fr.I deficient in EGF or TF was prepared by passing this fraction through an Affi-Gel Hz column coupled with anti-rat EGF or TF antibodies, respectively. EGF and TF eluted from the column (designated as eluted EGF and eluted TF) were also tested for tumor-enhancing activity. Fr.I passed through the column coupled with nonimmune rabbit IgG served as control (Fr.I column control). After initiation of carcinogenesis in HTBs by instillation of a single dose of 0.25 mg of N-methyl-N-nitrosourea, test materials were administered into these HTBs once a week for 30 weeks. The results showed that removal of EGF significantly reduced the tumor-enhancing effect of Fr.I (P less than 0.001 as compared to that of the Fr.I column control) and that eluted EGF by itself significantly enhanced the carcinogenesis as compared to that of the vehicle control (P less than 0.006). Removal of TF from Fr.I also reduced the tumor-enhancing effect of Fr.I (P less than 0.01). However, removal of both EGF and TF from Fr.I did not enhance the inhibitory effect demonstrated by the Fr.I which was deficient in EGF. Likewise, combined use of TF and EGF did not exceed the tumor-promoting effect of EGF. The results indicate that EGF in Fr.I may play a significant role in the promotion of bladder carcinogenesis by urine.