Several integral experiments on tritium breeding were jointly performed at the Fusion Neutronics Source (FNS) facility at the Japan Atomic Energy Research Institute (JAERI), in connection with the U.S./JAERI Collaborative Program on Fusion Breeder Neutronics. Tritium production rates from 6Li (T6) and 7Li (T7) were measured at several locations in an Li2O assembly (D = 60 cm, L = 60 cm) embedded in the concrete wall of a 5- × 5- × 4.5-m room (reference experiment). JAERI has also performed independent benchmark experiments with the Li2O assembly located in a large room of negligible room-return neutrons. In the reference experiment, large discrepancies in T6 were found at the front locations in the Li2O assembly. At middle locations, the calculated-to-experimental (C/E) values for T6 are ∼1.2 (U.S.) and ∼1.1 (JAERI). The C/E values for T7 are ∼1.18 (U.S.) and 1.05 (JAERI). To assess the contribution to the uncertainty in predicting T6 and T7 that results from the current uncertainties in the nuclear data base, an extensive two-dimensional cross-section sensitivity/uncertainty analysis was performed. For that purpose, the FORSS module, and the VIP and DOT 4.3 codes were used along with the PUFF-2 covariance code. Two systems were considered for the analysis: the benchmark system and the reference system. The models used simulate the geometrical details and source conditions for the experiments. After coupling the sensitivity profiles with the cross-section uncertainty information (ENDF/B-V, file 33), it was found that the standard deviations in T6 are 2.0 to 3.5%. In the reference system, the uncertainties in T6 at front locations due to data uncertainties were found to be very small (∼0.3%). The large discrepancies at these locations between the calculation and measurements were attributed to inaccuracy in modeling and predicting the room-return component of incident neutrons. The uncertainties in T7 due to the uncertainties in nuclear data were found to be 3 to 6%, with the largest values at back locations. The discrepancies with experimental values were attributed to the inaccuracy in the 7Li(n,n′α)t cross section, which requires further evaluation.