From human pregnancy serum we have isolated the proform of eosinophil major basic protein (proMBP), which forms a complex with pregnancy-associated plasma protein-A (PAPP-A), PAPP-A/proMBP. It is shown that proMBP contains O-linked glycan bound to Ser-24, Thr-25 (fully substituted), and to Thr-23 and Thr-34 (partially substituted). N-linked glycan is bound to Asn-86 and O-linked glycosaminoglycan is bound to Ser-62 (both fully substituted). From the RP-HPLC elution profile and mass spectra of tryptic peptides it is found that proMBP is extremely heterogeneous with respect to glycosylation.