The diallelic enzyme α-L-fucosidase has been found to be highly active in semen. Eight distinct enzyme bands were observed in each homozygote and the heterozygote consisted of a combination of bands. The isoelectric points ranged between pI 4.34 and 6.65. Activity was found to be much lower in vaginal fluid, not more than three cathodal bands were visible in any sample examined. In leucocytes, 5 bands were detected in homozygotes whilst in serum only two faint anodal bands could be detected. Activity was low in vaginal fluid and could not be detected in urine or saliva. Population studies were carried out on semen samples from Oxford and Basingstoke and on lymphocyte samples from Oxford. The data were in agreement with Hardy-Weinburg equilibrium. The common allele (Fu 1) had a frequency of 0.704 in Oxford semen samples