Small and marginal farmers with low livestock holdings do not strictly follow recommended vaccination schedules and procedures. The aim of this study was to follow antibody titers against Classical Swine Fever (CSF) virus before and after vaccination of cross-bred pigs from three farms in Khon Kaen. One dose of vaccine was given to the pigs at 41, 49, and 52 days age. It is a modifi ed lived vaccine containing 102 PD50 (50% protective dose) of the CSF virus (government produced Lapinized Chinese-strain, Department of Livestock Disease, Thailand). Blood samples were collected at 0, 14, and 22 days postvaccination (dpv). Neutralization test was performed in cell cultures using a constant-virus/varying-serum method. In Pigs born o sows that had been vaccinated once a year, geometric mean titers at 0 dpv of 41, 49, and 52 days old pigs were 2.05, 1.24, nd 0.72, respectively. The lowest/highest maternal antibody titers were 1:32/1:512, 1:8/1:64, 1:2/1:16, the median titers were 1:32, 1:16, and 1:8, in farms 1, 2, and 3, respectively. At 14 dpv, SN antibody titres of pigs in farm 1 decreased but that of farm 3

increased. Consequently, SN antibody titers of pigs in farms 2 and 3 increased after vaccination and had GMT of 1.82 and 1.09 (p<0.05) at 22 dpv, respectively. General vaccination guideline is not applicable to all farms. Higher SN titers at the day of fi rst vaccination impeded antibody response (14 days old). However, the lowest titer (1:8) and the oldest pigs (52 days old) did not have the best SN titer after CSF vaccination. There should be laboratory confi rmations of CSF antibody titers before and after vaccination in order to assure successful outcome of vaccination practice.