Four young male chimpanzees were inoculated intra-urethrally with a strain (G37) of Mycoplasma genitalium which had been isolated from the urethra of a patient with non-gonococcal urethritis. Two of the chimpanzees became infected as indicated by persistent recovery of the organisms from the urethra for 13 weeks and by an antibody response measured by both metabolism inhibition and micro-immunofluorescence techniques. The numbers of organisms isolated from both animals increased about 4 weeks after inoculation and antibody development was first detected 1 week later. The infected animals developed a minimal and inconsistently detected urethral polymorphonuclear leucocyte response which was not seen in those that were uninfected, nor in a chimpanzee that had been given medium only. The organisms were not isolated and the cellular response was not observed after treatment of the infected chimpanzees with oxytetracycline. One of the animals that had been infected was re-inoculated with strain G37 six months after successful treatment, but although the titre of serum antibody had diminished to its original level urethral recolonization did not occur. The organisms in the inoculum were not attenuated, however, because they infected another chimpanzee that had not had previous experience of M. genitalium. The results are discussed in relation to the potential of this mycoplasma to produce urethritis in man.