Immobilization of influenza viruses via their surface neuraminidase (NA) is much less established despite influenza NA inhibitors such as oseltamivir, zanamivir or peramivir displaying a much stronger monovalent interaction with NA than sialic acids binding to HA.6,7 We have designed phospha-oseltamivir–biotin conjugate 1, containing an undecaethylene glycol spacer moiety, which inhibits NA (from H3N2 X31 virus)8 in the subnanomolar range (Ki = 1.8 nM), similar to conjugate 2 containing only a short spacer (Ki = 0.24 nM) and only slightly weaker than oseltamivir itself (Ki = 0.12 nM). Here, XK is linked to influenza.