The primed in situ (PRINS) labeling technique was developed as an alternative method to classical cytogenetics and in situ hybridization (FISH) for the characterization of interspecific somatic hybrids. Full karyotypes were performed using Alu specific primers generating the painting of all human material associated with R like banding. The representativity of individual human chromosomes was established using primers specific for discriminent alpha-satellite DNA sequences providing specific signals on the centromeres of the targeted chromosomes and corresponding spots in interphase nuclei. Due to the use of synthetic oligonucleotide primers and of directly labeled haptens. PRINS method avoid repetitive probes preparation, eliminates secondary amplification of signals and the whole process can be performed within a timespan of 1 hour. Providing qualitative and quantitative answers, the simple PRINS method appears very well adapted to the specific problematic of somatic hybrids as for their characterization than for their periodic controls imposed by their instability. The method has been tested on 4 human-rodent hybrid cell lines. In particular, the somatic hybrid clone ALE 4 was shown to be monochromosomal for the der(11) from the reciprocal translocation t(11:22).