CEC kinetics and viability in melanoma- and breast cancer–bearing mice. Six-week-old mice received transplants of either a human melanoma cell line (MeWo) (A) or a metastatic breast cancer cell line (LM2-4) (B). Tumors were monitored on a weekly basis. When tumors reached 200 mm3, mice were treated with 0, 10, 20, or 40 mg/kg/d of cyclophosphamide (black, red, green, and blue, respectively), or 0, 3, 6, 9, or 12 mg/kg 3 times a week of vinorelbine (black, red, orange, green, and blue, respectively). After 1 week of treatment, mice were bled from the retro-orbital sinus and blood was assessed for the levels of apoptotic CECs (panels C and D, respectively). Panel E shows apoptotic CEC counts in nontumor-bearing BALB/c mice treated with CTX for 1 week. The reduction in CEC count observed in mice treated with 50 mg/kg/d CTX is most likely due to the severe myelosuppression described in this same model by Shaked et al.17 *P > .05 indicates nonsignificant; **P < .05 indicates statistically significant from control using t test statistical analysis. Results are expressed as mean ± SD.