Freezability of stallion spermatozoa varies considerably between individuals. The aim was to investigate whether adding seminal plasma (SP) from good freezer or bad freezer stallions improved post-thaw sperm acrosomal integrity. Semen samples were available where SP-free spermatozoa from good freezer and bad freezer stallions prepared by SingleLayer Centrifugation (S) were treated with pooled SP from the same stallions prior to freezing. Control samples were frozen conventionally (C). Post-thaw acrosomal integrity was assessed by flow cytometry with/without stimulation by calcium ionophore. Higher proportions of live acrosome-intact spermatozoa (p = 0.04) and lower proportions of dead acrosome-intact spermatozoa (p = 0.48) occurred in non-stimulated S samples than in C. Acrosome integrity was not affected by either type of SP. However, in ionophore-stimulated samples, SP from good freezer stallions produced higher proportions of live acrosome-intact spermatozoa than C (p = 0.047), whereas SP from bad freezer stallions resulted in higher proportions of live acrosome-reacted spermatozoa than C (p = 0.04); the proportion of dead acrosome-intact spermatozoa was higher in C than in S (p = 0.05). Thus, S had a lower propensity to acrosome-react under stimulating conditions than C; the effect of adding SP depends on its origin. Further research is required to optimise cryopreservation protocols for stallion semen.