The polymorphism of DIA3 was investigated by isoelectric focusing in semen samples from 235 unrelated Japanese volunteers and patients. Besides the three common phenotypes seven samples of the type 3-1 were observed. However, readable isoenzyme patterns were not demonstrated in semen samples of oligospermia under about 10 X 10(6)/ml sperm cells. The allele frequencies were DIA3*1 = 0.821, DIA3*2 = 0.164, and DIA3*3 = 0.015. The DIA3*1 frequency in oligospermia (0.765) was lower than that in normospermia (0.836). The isoelectric focusing method was successfully applied to phenotyping DIA3 in seminal stains; each phenotype was demonstrated at 37 degrees C for up to 4 weeks, at room temperature for up to 8 weeks, and at 4 degrees C for over 12 weeks after stain formation. In vaginal swabs the isoenzyme bands were very faint and not identifiable.