R802, a new synthetic quinolone derivative, has been investigated for in-vitro activity against clinical isolates from patients with urinary tract infection and for in-vivo efficacy in the management of urinary tract infection. Of 835 urinary isolates, 88·9% were R802-sensitive and of 731 isolates of Enterobacteriaceae 97·7% were sensitive. R802 was active against 1 5 (55%) of 27 nalidixic acid-resistant isolates. Twenty-six adult females with urinary tract infection were treated with R802. Urinary concentrations of R802 exceeded the MIC of the infecting organisms in all patients. Short-term eradication of bacteriuna was achieved in 22 (85%). However, in three of four patients with persistent infection the organisms became resistant during R802 therapy and in two patients an R802-resistant enterococcal infection supervened. Seventeen of 20 patients with satisfactory short-term eradication of bacteriuria were followed-up, seven remaining abacteriuric for 6 months and two for shorter periods. R802 was well tolerated and a significant adverse reaction (skin rash) was seen in only one patient. Further studies of R802 in urinary tract infection appearjustified.