Human placental sphingomyelinase was highly purified through an original six-step scheme in order to raise a specific rabbit anti-sphingomyelinase antibody. Pure enzyme preparations showed specific activities ranging between 100 and 150 mumol/h per mg protein and gave two constant silver-stained bands (Mr 70,000 and 57,000) on acrylamide after electrophoresis under denaturing conditions. These two bands were the sole areas detected by the described antibody on Western blots from normal placental preparations at various stages of purification. A similar procedure was applied to the separate study of placental sphingomyelinase from two prenatally diagnosed foetuses with confirmed Niemann-Pick disease type A. During purification, the mutant enzyme could be followed owing to its minute but measurable level of catalytic activity, and behaved normally at the various chromatographic steps. In the purified preparations, specific activities of 0.18 and 0.49 mumol/h per mg protein, respectively, were reached. No alteration of the Km value (19 mumol/l) was observed, while the Vmax was 0.5-1% of normal. With immunostaining of Western blots obtained as above, results similar to those described for normal tissue were found, leading to the conclusion that immunoreactive sphingomyelinase is present in Niemann-Pick disease type A.