A series of experimental and theoretical investigations on neutron spectra in lattice cells has been started at the reactor R1. This report gives the results from the work on the first three cells. The cells consisted of uranium tubes surrounded by D2O. They were placed in the central vertical channel of the reactor. The neutron spectra from lead scatterers in the center of the tubes were measured with a fast chopper in the energy region from 0.01 to 100 eV. The beam spectrum from the lead piece was expected to correspond to the angular flux integrated over all angles. This was tested in an accessory experiment which gave some correction factors for the tube measurements. For the calculations the THERMOS code has been used. The energy region ranged from 0.00025 to 3.06 eV. Three scattering models for deuterium have been used—the free-gas model (D mass 3.595), the effective-width model, and the Nelkin model. With all three models the THERMOS code reproduced the thermal spectra within a few percent. The effective-width model and the Nelkin model also gave reasonable agreement in the epithermal region. The free-gas model, however, gave too high a ratio between the epithermal and the thermal neutron densities. The computation time required for a 20-space, 30-energy-group calculation on an IBM-7090 computer was about 4 min.