We have studied the 5′ flanking region of the ACP1 gene looking for new polymorphisms. Two SNPs, DdeI and FokI restricted, have been found in this region. We determined the genotype of DdeI and FokI SNPs, as well as of three other known SNPs, codon 43 (CfoI restricted), codon 41 and codon 105 (TaqI restricted), located respectively in exons 3, 4 and 6 in 62 unrelated subjects from the Italian population. Haplotype distribution for the ten possible pairs of loci were determined by a maximum likelihood procedure. Overall, statistically significant deviations from expected frequencies assuming equilibrium have been observed for the following pairs: FokI/codon 41, FokI/TaqI, codon 41/TaqI (complete association), DdeI/FokI, DdeI/codon 41 and DdeI/TaqI. The data suggest that the FokI area could include sequences operating in strict functional association with sequences included in the codon 41/TaqI area, possibly in order to regulate the F/S isoforms ratio of the A* and *B alleles. Since the ratio between the concentration of the two F and S isoforms is different for the three ACP1 alleles, we suggest that the ACP1 locus has been subjected to strong selective pressure to obtain an optimal alternative splicing mechanism of the *A and *B alleles. The *C variant, on the other hand, seems completely independent from sequences in the FokI/codon 41/TaqI areas, resulting in an inverted F/S ratio compared to that found for *A and *B alleles.