Elliptocytogenic alpha I/36 spectrin Sfax is a new variant found in a Tunisian family. The alpha I/36 allele yielded a clinically manifest picture only when occurring in trans to a recently identified, low expression level polymorphism referred to as the alpha V/41 allele. Spectrin dimers were slightly increased in 4 degrees C extracts. On peptide maps, the alpha I domain split into two abnormal fragments of 36 and 33 Kd. The mutated alpha-chain represented 20% and 44% of total alpha-chain in alpha/alpha I/36 and alpha V/41/alpha I/36 heterozygotes, respectively. Peptide sequencing showed that the 36-Kd fragment started at Ala 357 and displayed a deletion extending from amino acids 363 to 371. The corresponding 27-nucleotide deletion was found in alpha-spectrin mRNA. However, exon 8 of spectrin alpha-gene failed to disclose this deletion. Instead, an A to G substitution appeared in position 3 of codon 362, leading to the occurrence of the critical GU dinucleotide within a cryptic 5'-splice site surrounding codon 362. This event would account for the splicing out of codons 363 to 371. The reading frame was preserved and even amino acid 362 (AGG, Arg) remained unaltered. As in most spectrin alpha-chain elliptocytogenic variants, the change involved a helix 3. This is the first elliptocytogenic mutation recorded in repeat alpha 4.