The effects of Intralipid (IL) administration of preterm infants have been investigated. For this purpose, adult neutrophils were stimulated by infant sera collected before and after IL treatment. A definite decrease (p less than 0.0005) in chemoattractant capacity of zymosan-activated serum was observed after IL. On the other hand, no difference in the opsinizing abilities of untreated versus treated sera could be demonstrated by measuring the chemiluminescence responses to serum-opsonized zymosan. This study emphasized the need of caution in the use of IL treatment for preterm infants.