The adjuvanticity of a sulfolipopolysaccharide (SLP) incorporated into a squalane-in-water emulsion (SLP/S/W) was compared with that of a mineral oil-in-water (O/W) adjuvant currently used in commercial porcine vaccines. Groups of pigs were immunized twice with vaccines comprising either inactivated influenza virus (iFlu3 containing strains A/Swine, MRC-11 and X-79), inactivated pseudorabies virus (iPRV), live pseudorabies virus (PRV) or inactivated porcine parvovirus (iPPV) as antigen and SLP/S/W or O/W as adjuvant. Antibody titres in serum 2 or 3 weeks after the second immunization were measured by haemagglutination inhibition (HI) or serum neutralization (SN) assays. Both adjuvants significantly augmented the antibody responses against the antigens tested. Mean factors of increase obtained by SLP/S/W and O/W were: 315 and 91, respectively, for A/Swine; 478 and 137 for MRC-11; 362 and 128 for X-79; 69 and 49 for iPRV; and 23 and 7 for live PRV. Increased humoral immunity against live PRV was affirmed by reduced levels and duration of virus excreted by pigs after challenge with virulent PRV. Immunization of pigs with iPPV plus adjuvant SLP/S/W gave 36-fold higher titres than with O/W. It was concluded that SLP/S/W is more effective than O/W in stimulating humoral immunity against the viral antigens examined and that the two constituents SLP and S/W interact synergistically. Advantages of SLP/S/W over O/W include stronger adjuvanticity, better biocompatibility and lower doses of active substances.