Platinum microquantities were determined in plasma of patients affected by lung carcinoma during treatment with radiotherapy (RT) and concurrent low-dose continuous infusion of cis-dichlorodiammineplatinum(II) (CDDP). RT was given at 50 Gy in continuous course; CDDP was continuously infused at 4 mg/m2 daily for 100 h/week for 5 weeks, and the infusions were separated by 68 h of rest. The percentage of free drug versus total drug in plasma was about 3%. It did not vary with therapy duration and was not significantly different from that found in 5-day continuous infusions at much higher daily doses. Nevertheless, maximal values of free Pt in plasma were very low and agreed with the low level of CDDP toxicity encountered on the present administration schedule.