The function and chilling sensitivity of boar spermatozoa differ from other domestic animals. We hypothesised that exogenous lipids added to sperm membranes would influence the viability, motility, in vitro capacitation and acrosome reaction of boar sperm before and/or after cryopreservation. Lipids in the form of liposomes were fluorescently labelled and their fusion to spermatozoa was quantitated by flow cytometry. Liposomes of selected lipids (SLM), at 261.3 [mu]g/ml in BTS buffer, fused with >90% of sperm in a stable and repeatable fashion and did not affect in vitro capacitation and acrosome reaction of fresh sperm, but improved viability (p < 0.05). Liposomes from egg yolk lipids at 261.3 and 1378.2 [mu]g/ml (EYA and EYB, respectively) fused to fewer sperm than SLM (p < 0.05). During cooling and after thawing, spermatozoal viability and motility were best protected by extender containing whole egg yolk ± liposomes > EYB > SLM > EYA.