An early identification of fusion products was based on the presumed vigorous growth of hybrid calluses after fusion between Solanum brevidens and S. tuberosum leaf protoplasts. The S. brevidens protoplasts were unable to form multicellular colonies under the applied culture conditions. Three size groups of calluses were separated and analyzed at two different early phases of culture period. "Squash blot" hybridization with a S. brevidens specific repetitive DNA probe showed that the group of the largest calluses consisted of putative somatic hybrids with a frequency of 80-100% in three independent experiments. Furthermore, approximately 80-95% of the middle sized calluses and 33-90% of the smallest ones were shown to be hybrid. The unexpectedly high percentage of fusion products, even in the case of the smallest calluses, may result from the suppression of the development of parental potato colonies in cultures with mixed cell population. Till this time 120 independent colonies selected as putative hybrids have been regenerated into plants. All of them exhibited hybrid phenotype, and their hybrid origin was proved by cytological and restriction fragment length polymorphism analyses.