CI-921 (NSC 343499; 9-[[2-methoxy-4-[(methylsulphonyl)amino]phenyl]amino] -N,5-dimethyl- 4-acridinecarboxamide) is a topoisomerase II poison with high experimental antitumour activity. It was administered by 15 min infusion to 16 evaluable patients with non-small cell lung cancer (NSCLC) (7 with no prior treatment, 9 patients in relapse following surgery/radiotherapy) at a dose (648 mg/m2 divided over 3 days, repeated every 3 weeks) determined by phase I trial. Patients had a median performance status of 1 (WHO), and median age of 61 years. The histology comprised squamous carcinoma (11), adenocarcinoma (1), mixed histology (2), bronchio-alveolar carcinoma (1) and large cell undifferentiated carcinoma (1). Neutropenia grade greater than or equal to 3 was seen in 15 patients, infections with recovery in 3, and grand mal seizures in 1 patient. Grade less than or equal to 2 nausea and vomiting occurred in 66% courses and phlebitis in the infusion arm in 37%. 1 patient with squamous cell carcinoma achieved a partial response lasting 5 months. Further testing in this and other tumour types using multiple daily schedules is warranted.