Prenatal diagnoses of I-cell disease were carried out by examination of the amniotic fluid and cultured amniotic cells in four cases of high-risk pregnancy in three different families. Three of the four fetuses were diagnosed as having I-cell disease, on the basis of observation of abnormally increased activity of lysosomal acid hydrolases in the amniotic fluid and their decreased activity in cultured amniotic cells, thus leading to therapeutic abortion. alpha-Mannosidase in the amniotic fluid of the fetuses with I-cell disease exhibited significant alterations in pH profile Km value, thermal stability and isoelectric focusing pattern, compared with the enzyme in normal controls. The results indicate that prenatal diagnosis of I-cell disease may be accomplished by demonstration of altered enzymological characteristics of alpha-mannosidase in the supernatant of amniotic fluid. The significance of the alteration of alpha-mannosidase in the amniotic fluid is not known.