Transcobalamin II (TC II) deficiency is a rare autosomal recessive disease leading to cobalamin (Cbl; Vitamin B12) deficiency characterized by failure to thrive, megaloblastic anemia, impaired immunodefence and neurological manifestations. By means of Southern blotting and sequence analysis of TC II cDNA amplified from fibroblasts of an affected child and his parents, we have identified two mutant TC II alleles, one with a gross deletion and the other with a 4 nucleotide deletion. Both the mutations caused TC II mRNA and protein deficiency and hence defective plasma transport of Cbl and the development of Cbl deficiency in the affected child. The present study has identified molecular defects that cause TC II deficiency and lead to intracellular Cbl deficiency in humans.