Cowpox virus (CPV) expresses the serpin (serine proteinase inhibitor) CrmA, an anti-inflammatory, anti-apoptotic protein required for production of red pocks on chicken chorioallantoic membranes (CAMs). In vitro, CrmA inhibits several caspases and granzyme B. Altering the critical P1-aspartate in the CrmA reactive centre loop to alanine resulted in a virus (CPV-CrmA-D303A) that resembled CPV deleted for CrmA (CPVDeltaCrmA : : lacZ); on CAMs it produced white, inflammatory pocks with activated caspase-3 and reduced virus yields, suggesting that CrmA activities are mediated via proteinase inhibition. CrmA in CPV was replaced with SERP2 from Myxoma virus (MYX) or baculovirus P35, which inhibit similar proteinases in vitro. SERP2 and P35 each blocked caspase-3-mediated apoptosis but were unable to control inflammation of CAMs. However, SERP2 and P35 restored virus yields, indicating that the decreased virus titres seen with CPVDeltaCrmA : : lacZ resulted from apoptosis rather than inflammation. To compare the activities of CrmA and SERP2 further, rabbits were infected with MYX recombinant viruses. Intradermal infection of rabbits with MYX was uniformly lethal, generating raised primary lesions and many secondary lesions. In contrast, deletion of SERP2 from MYX (MYXDeltaSERP2 : : lacZ) caused little mortality and produced flat primary lesions with few secondary lesions. Replacement of SERP2 with CrmA (MYXDeltaSERP2 : : CrmA) resulted in partial complementation with flat primary lesions, many secondary lesions and death in 70 % of the rabbits. Therefore, CrmA and SERP2 were not functionally interchangeable during infection of CAMs or rabbits, implying that these serpins have activities that are not evident from biochemical studies with human caspases.