Results of investigations on the quality of string-been thrashing with a model thrashing crushing-wiping unit have been presented. Analysis of the a.m. results confirmed that the size and type of seed losses in the thrashing process of dried string-bean seeds var. Presenta depend on the construction parameters of the thrashing concave, size of the working slot /crack/ and moisture level of the seeds thrashed. It was observed that with moisture increase in the range studied, seed damage decreases but at the same time a percentage of un-thrashed seeds increases. The level of these latter losses increases also with an increase of the working slot size, and their level depends on the activity of the thrashing concave. A trace amount of broken pod segments in the mass sieved through section concaves confirmed that this type of losses can be effectively decreased by an adjustment of the concave sieving efficiency to the seed and pod size of the variety thrashed. A low level of seed losses, and especially a low level of seed damage during thrashing, showed that the unit examined constitutes a marked progress in the search for a rational design of the string-bean thrashing unit.