The molecular genetics of the P blood group system and the absence of P1 antigen in the p phenotype are still enigmatic. One theory proposes that the same gene encodes for both the P1 and Pk glycosyltransferases, but no polymorphisms in the coding region of the P k gene explain the P1/P2 phenotypes. We investigated the potential regulatory regions up- and downstream of the A4GALT (P k ) gene exons. P1 (n = 18) and P2 (n = 9) samples from donors of mainly Swedish descent were analysed by direct sequencing of PCR-amplified 5'- and 3'-fragments surrounding the P k coding region. Seventy-eight P1 and P2 samples were investigated with PCR using allele-specific primers (ASP) for two polymorphisms previously proposed as P2-related genetic markers (-551_-550insC, -160A>G). Haplotype analysis of single nucleotide polymorphisms was also performed with PCR-ASP. In ~1.5 kbp of the 3'-untranslated region one new insertion and four new substitutions compared to a GenBank sequence (AL049757) were found. In addition to the polymorphisms at positions -550 and -160, one insertion, two deletions and one substitution were found in ~1.0 kbp of the 5'-upstream region. All 20 P2 samples investigated with PCR-ASP were homozygous for -550insC. However, so were 18 of the 58 P1 samples investigated. Both the 20 P2 and the 18 P1 samples were also homozygous for -160G. The proposed P2-specific polymorphisms, -551_-550insC and -160G, found in P2 samples in a Japanese study were found here in homozygous form in both P1 and P2 donors. Since P 2 is the null allele in the P blood group system it is difficult to envision how these mutations would cause the P2 phenotype. None of the novel polymorphisms reported in this study correlated with P1/P2 status and the P1/p mystery remains unsolved.