Human Serum Cholinesterase activity and polymorphism at its two gene loci, CHE1 and CHE2, were compared in maternal serum from neural tube defect pregnancies, normal pregnancies and a non-pregnant control group. Variants at the CHE1 locus were identified by dibucaine, fluoride and R02 0683 inhibition. The CHE2 phenotype was demonstrated by DISC polyacrylamide gel electrophoresis. Total HSChE activity in the pregnant groups was slightly less than in the control group but there was no difference in activity between the affected and the normal pregnancies. Three variants were identified. All were found in the non-affected pregnant group. One variant at CHE1 was identified, an I phenotype, and two C5+ phenotypes, the CHE2 variant. No obvious relationships were found between HSChE activity or a particular genetic variant and NTD progeny.