Groups of inbred mice were inoculated intraoesophageally with Giardia lamblia cysts isolated from the stools of 15 patients with various clinical conditions: symptomatic diarrhoeic, symptomatic non-diarrhoeic and asymptomatic cyst carriers. The virulence of these isolates was studied in terms of two-hour cyst excretion rate and trophozoite counts in the duodenum. The basic pattern of the cyst excretion was similar in all the groups: cysts appeared in the stools by Day 3 +/- 1, reaching a maximum by Day 11 +/- 2. There was then a gradual decline until Day 23 +/- 2, with a low output until Day 31 post-inoculation. However, statistically significantly higher numbers of cysts were excreted by animals which had received cysts isolated from symptomatic diarrhoeic patients than by mice which received cysts from symptomatic non-diarrhoeic or asymptomatic cases. Identical results were obtained with the trophozoite counts. Haemagglutinating (HA) antibodies appeared after Day 11 post-inoculation in each group regardless of different cyst isolates, and the HA titre increased until Day 31 post-inoculation. The results indicate that variable symptomatology in patients could be due to variation in the virulence of the parasite.