Our objective was to extend the linkage and comparative maps of BTA23 by determining whether the structural gene for the A subunit of blood coagulation factor XIII (F13A) is linked to BoLA-DYA, a centromeric marker, or the distally located gene encoding prolactin (PRL). Bovine F13A was mapped relative to DYA and PRL in an experiment that examined segregation of alleles in 176 sperm. Genotyping was performed by PCR-RFLP for all loci, following amplification of the haploid genome by primer extension preamplification. F13A was found to be linked to PRL (theta = 0.314 +/- 0.038). The most likely order is DYA-PRL-F13A (odds > 10(4):1). This result demonstrates conservation of synteny between BTA23 and most of HSA6p. Surprisingly, theta DYA-PRL was 0.310 +/- 0.039, 83.4% greater (P < 0.02) than we found for another bull (Van Eijk et al., Mamm. Genome 4: 113, 1993). The difference in recombination rate in the DYA-PRL interval provides further evidence for an unusual recombination hot spot between the bovine Mhc class IIa and class IIb subregions and suggests that bull-specific maps may be necessary for marker-assisted selection.