A new technique is described for amplifying individual alleles in a mixture of two or more alleles by the polymerase chain reaction (PCR) to determine their nucleotide sequence. This technique involves amplifying and separating target sequences by the PCR-mediated single-strand conformation polymorphism (PCR-SSCP) method, isolating each polymorphic DNA strand, and amplifying it by a second-stage PCR for its sequence determination. By this technique, the sequence of a minor constituent (approximately 3%) can be determined accurately.