A multivalent lacto-N-fucopentaose (LNFP) III-lysyllysine conjugate was observed to decompact preimplantation mouse embryos. Decompaction was not obtained with free oligosaccharides (LNFP II and III), nor with multivalent LNFP II-lysyllysine or chitotriose-lysyllysine conjugates. These results suggest a role for X hapten recognition during compaction and suggest further that X hapten valency may play a key role in modulating this developmental process.