The in vitro cultivation of human cells is an essential part of the work of every diagnostic cytoge-netics laboratory. Almost all human cytogenetic studies involve the examination of dividing bloodcell population by blocking cell division at metaphase with subsequent processing and staining bybanding techniques. The chromosome constitution is described as Karyotype that states the totalnumber of chromosomes and the sex chromosome constitution. Karyotypes are prepared by cuttingup a photograph of the spread metaphase chromosomes, matching up homologous chromosomes andsticking them back down on a card or nowadays more often by getting an image analysis computerto do the job. Chromosomes are identied by their size, centromere position and banding pattern.Teaching a student how to detect and interpret even the most common chromosome abnormaliti-es is a major challenge: mainly, in a developing country where the laboratorial facilities are notalways available for a big number of students. Therefore, in this work we present an educationalsoftware for teaching undergraduate students of Medical and Life Sciences Courses how to arrangenormal and abnormal chromosomes in the form of karyotype. The user, using drag-and-drop, is da-red to match up homologous chromosome. For that, we have developed a free full access web site(http://www.biomol.net/cariotipo/) for hosting the software. The latter has proved to be light andfast even under slow dial-up connections. This web site also oers a theoretical introductory sectionwith basic concepts about karyotype. Up to now the software has been successfully applied to un-dergraduate courses at the University of Rio de Janeiro (UNIRIO). The students have approved thesoftware; to them the similarities with the well-known game solitaire turns the exercise more excitingand provides additional stimulus to learn and understand karyotype. Professors have also used thesoftware as complementary material in their regular classes. Based on their testimonies the softwarewas a helpful tool for explaining the karyotype assembly.