Exclusion mapping was applied to 10 cases of unbalanced chromosomal rearrangements involving 12 different monosomic segments. A range of genetic markers and blood groups were determined. New exclusions arising from the detection of heterozygosity at codominant loci were detected for the unassigned markers at the following segments: (3)(p25→pter), GPT, PI; (4)(q27→31), MNS, JK, PI, C3, F13B; (7)(q22), MNS, LU, GPT, F13A;(12)(p12), MNS; (12)(p13) and (15)(q15), GPT. C3; (12)(pter) and (19)(pter) or (19)(qter), MNS, GPT, PI: and (18)(q21→23), MNS, JK. These exclusions will assist regional localization if the loci are subsequently assigned to the chromosome involved in the exclusion. No new gene assignments were made from the cases where family data were obtained. Exclusion mapping was applied to 10 cases of unbalanced chromosomal rearrangements involving 12 different monosomic segments. A range of genetic markers and blood groups were determined. New exclusions arising from the detection of heterozygosity at codominant loci were detected for the unassigned markers at the following segments: (3)(p25→pter), GPT, PI; (4)(q27→31), MNS, JK, PI, C3, F13B; (7)(q22), MNS, LU, GPT, F13A;(12)(p12), MNS; (12)(p13) and (15)(q15), GPT. C3; (12)(pter) and (19)(pter) or (19)(qter), MNS, GPT, PI: and (18)(q21→23), MNS, JK. These exclusions will assist regional localization if the loci are subsequently assigned to the chromosome involved in the exclusion. No new gene assignments were made from the cases where family data were obtained.