Heparan sulphate and sialoadhesin were previously identified on porcine macrophages as receptors for porcine reproductive and respiratory syndrome virus (PRRSV). In this study, the exact role and cooperation of heparan sulphate and sialoadhesin during PRRSV attachment and internalization was analysed. It was observed that both heparan sulphate and sialoadhesin mediate PRRSV attachment and that only these two receptors are involved in attachment. Analysis of attachment kinetics of PRRSV to macrophages revealed that early attachment is mediated mainly via an interaction with heparan sulphate, followed by a gradual increase in interaction with sialoadhesin. By using wild-type CHO and CHO deficient in heparan sulphate expression, it was shown that heparan sulphate alone is sufficient to mediate PRRSV attachment, but not entry, and that heparan sulphate is not necessary for sialoadhesin to function as a PRRSV internalization receptor, but enhances the interaction of the virus with sialoadhesin.