This study investigates the immunomodulatory effect of a C3 binding glycoprotein (C3bgp), isolated from the parasitic plant Cuscuta europea. When BALB/c mice, immunized with sheep red blood cells (SRBC), were given a single intraperitoneal injection of C3bgp a dose-dependent immunostimulation was observed. The stimulation was assessed by an increase in the number of haemolytic plaque forming cells (PFC) and haemaglutination titres. The induction was time dependent in respect to the administration of both the C3bgp and SRBC. When C3bgp was applied 24 h before SRBC at a dose of 30 microg per mouse (1.2 mg/kg), a well expressed immunostimulation was found. It was also found that giving C3bgp to mice, which had previously been treated with the immunosuppressive drug cyclophosphamide (CY), produced an increase in PFC. The immune response was also restored in vitro experiments were performed using human whole blood cultures stimulated with 30 microg/ml C3bgp in the presence or absence of egg albumin (OVA) as antigen for 72 to 168 h. In C3bgp stimulated cultures it was found that after 120 h there was a high expression of the CD 19+ subset of the activation antigen CD25 (IL-2R) as assessed by flow cytometric phenotype analysis. Supernatants from cultures with different stimuli were assayed by a solid phase ELISA for the determination of OVA-specific IgM at 120, 144 and 168 h. It was found that C3bgp application alone, failed to enhance OVA specific IgM, but significantly high levels of IgM in cultures containing C3bgp and OVA, were detected. Overall it has been shown that the C3 binding glycoprotein, as obtained from the parasitic plant Cuscuta europea, has strong immunostimulatory properties both in vivo and in vitro.