The present paper investigates the use of G-bands chromosome heteromorphisms for illegitimacy testing. We studied both parents and 171 of their children in 80 families utilizing the material, in addition, to establish the procedures sensitivity. The results showed that one (0.6%) of the 171 children studied was illegitimate, which was much lower than the figure of 6.7% obtained in a similar population studied with three blood group systems and three serum genetic markers. We could show that the low efficiency of the G-band heteromorphisms is at least partially due to its low sensitivity, 27% as compared to 60% obtained with the other genetic markers.