Tumor progression to the metastatic phenotype is accompanied in certain cell types by reduced expression of the nm23 gene. We have localized human nm23-H1 to chromosome 17 by somatic cell hybrid analysis. Regional localization in the CEPH database and in situ hybridization is reported. Somatic allelic deletion of nm23-H1 was observed in human breast, renal, colorectal, and lung carcinoma DNA samples, as compared to DNA from matched normal tissues. A homozygous deletion of nm23-H1 was observed in a lymph node metastasis of a colorectal carcinoma, indicating that nm23-H1 can be recessively inactivated. The data identify nm23-H1 as a novel, independent locus for allelic deletion in human cancer, a characteristic shared with previously described suppressor genes.