The single-strand conformation polymorphism (SSCP) technique was used to detect carriers of the known point mutation in the first exon of the porphobilinogen deaminase gene in Finnish and Swedish families. The SSCP technique was a reliable and convenient way of distinguishing patients from healthy members in a family. This point mutation is thought to result from a splicing defect of the mRNA. The PCR-based analyses of a patient's cDNA did not reveal the presence of an abnormal mRNA population, suggesting that no abnormal mRNA is synthesized or that it is too unstable to be detected.