To date, the only known apolipoprotein B (apo B) mutation causing hypercholesterolemia is the apo B 3500 Arg-->Gln or the familial defective apo B (FDB) mutation. This mutation has not been detected in the Finnish population. We have set up a systematic single-strand conformation polymorphism (SSCP) analysis-based screening method to search for other mutations in the exon 26 of the apo B gene in 21 Finnish hypercholesterolemic probands. The 7572-bp exon 26 covers half of the coding region of the gene including the DNA sequence coding for the putative low-density lipoprotein (LDL) receptor binding site on the apo B protein. Exon 26 was amplified as six 1190- to 1435-bp fragments, each of which was further split into three smaller 213- to 579-bp segments by restriction enzymes. These digestion products were run on nondenaturing polyacrylamide gels using at least three different electrophoretic conditions and autoradiographed. All previously known genetic variants in the exon 26 were detected by the SSCP method. A C-->T change at nucleotide 7064, in complete association with the XbaI site, was characterized by direct sequencing. This variant did not affect the amino acid sequence of the apo B protein. The SSCP-based procedure appears suitable for systematic screening for DNA sequence changes in large coding regions.