The MVM of rat small intestine undergoes progressive desialylation and fucosylation during postnatal development. In this study, we assessed the role of this process on the terminal reactivities of MVM glycans. Purified MVM of 3-21 day-old and weaned rats were delipidated and treated with 1 u of neuraminidase (Clostridium perfrigens) or 0.5 u of fucosidase (bovine epididymis). The delipidated-desialylated (DDSM) and delipidated-defucosylated (DDFM) membranes (4.0 μg protein) were incubated with 125I-labeled lectins (peanut, wheat germ, Ulex I), and the bound radioactivity measured by collecting the membranes on Millipore filters. Nonspecific binding was measured in the presence of 0.15 M specific sugars. From suckling to weaning periods: a) the binding of WGA to neuraminidase-sensitive sites decreased by 40% (p<0.01), while that of UEAI to fucosidase-sensitive sites increased by 13-fold (p<0.001); b) the binding of PNA to DDSM fell by 51% (p<0.02), while binding to DDFM rose by 3.7-fold (p<0.01). Next, the membrane preparations were treated with pronase (30 u), the supernates (30,000 g x 20′) applied to PNA or WGA affinity colums, and the bound proteins eluted with 0.2 M lactose or N-acetylglucosamine, respectively. By SDS-PAGE, the main protein fractions were in the MW region of the mucin-type glycoproteins (260-340 K). Thus, the lectin binding properties of O-glycans in mucins of rat MVM seem to be developmentally controlled through a shift from terminal sialylation to fucosylation.