In a randomized multi-institutional trial of the Eastern Cooperative Oncology Group, 316 patients with advanced measurable colorectal adenocarcinoma were treated with a weekly schedule of 5-fluorouracil given orally and intravenously with oral-5-fluorouracil in combination with cyclophosphamide or 6-thioguanine, or with oral Methyl CCNU administered once every eight weeks. On failure or progression, 133 protocol patients crossed-over to a secondary therapy, while 116 other patients previously treated with 5-fluorouracil off protocol were randomized to treatment with Methyl CCNU or B-2'-deoxythioguanosine. Response rates among patients who had received no prior chemotherapy were 18% to oral 5-FU, 15% to intravenous 5-FU and to MeCCNU, 12% to 5-FU and 6-thioguanine and 5% to cyclophosphamide and 5-FU, with little activity (3% response rate) in crossover or previously treated patients. Treatment with 5-FU, particularly oral 5-FU was associated with the least drug-related toxicity. Hematologic toxicity was greatest with Methyl CCNU, but was no more frequent in previously treated than in untreated patients. A tendency toward cumulative bone marrow depression was noted. 5-FU was effective only in ambulatory patients, whereas responses among non-ambulatory patients were seen only in the group treated with Methyl-CCNU.