Following a 2-year study, the combination of oral ciprofloxacin and colistin has been used continuously for 10 years without the emergence of resistance. During a 2-year period (1987-1989), we compared ciprofloxacin + colistin (CIP + COL) with neomycin + colistin (NEO + COL) in a randomized trial--combinations chosen because of the potential for prophylaxis of Gram-negative infection by ciprofloxacin, with colistin given to reduce the risk of emergence of resistance. Sixty-four patients with similar demographics in each arm were evaluable for efficacy analysis. Patients on CIP + COL had a significantly lower proportion of neutropenic days with fever (P < 0.001) and neutropenic days on intravenous antibiotics (P < 0.001) than patients on NEO + COL. A total of 54 (15 bacteriologically documented) pyrexial episodes occurred in patients on CIP + COL and 77 (41 bacteriologically documented) in patients on NEO + COL. Only two Gram-negative bacterial infections occurred in the CIP + COL arm compared with 16 in the NEO + COL arm. No Staphylococcus aureus infections occurred in the CIP + COL group compared with 10 in the other patients. Two CIP-resistant Gram-negative bacilli were isolated from patients on CIP + COL compared with 13 NEO-resistant Gram-negative bacilli from patients on NEO + COL. Following a subsequent decade of unchanged use of this prophylactic strategy in neutropenic patients, a 2-year follow-up study between 1 January 1998 and 31 December 1999 showed 66 significant infections during 700 [corrected] neutropenic episodes. Thirty-five of the 111 (31%) isolates were ciprofloxacin-resistant, involving 5% of the neutropenic episodes [corrected].