The purpose of the study was to determine the I.D.(50) for Giardia lamblia (CDC:0284:1) cysts in Mongolian gerbils (Meriones unguiculatus) and compare it to human infectivity data. Cysts were purified from Mongolian gerbil feces and diluted to produce inocula for each dosage group. After observation of the gerbils for production of either cysts in the feces or trophozoites in the small intestine over a two week period, the I.D.(50) results calucated by Probit anaysis, Spearman-Karber, and Reed-Muench were 282, 288, and 219 cysts, respectively. A least squares regression analysis, relating the number of cysts inoculated and the percent of infectivity, estimated the log(10) I.D.(50) to be 2.50, with a coefficient of correlation (r) of 0.9689. The log(10) I.D.(50) derived by this method is similar to those calculated by the previous methods. Since human feeding studies indicate the I.D.(50) for humans is < or = 10 and the authors results indicate the I.D.(50) for gerbils to be around 285 cysts, the gerbil model can not be trusted to reflect what might happen in human exposures.