A laccase gene from Trametes sp. 420 was recombinantly expressed in Pichia pastoris, producing the enzyme rLacD. Six mutant enzymes were produced by site-directed mutation at six potential glycosylation sites in the enzyme rLacD respectively. To probe the mutants with lower activities sensitively and specifically, the antiserum containing specific polyclonal antibodies were prepared by immunizing healthy male rabbits, about 4-month-old and 2 kilogram weight, using pure rLacD as an immunogen. Antibodies were collected after the fifth immunization injection. The antiserum had titres of 1:32 in double immunodiffusion test and of 1:128,000 in enzyme-linked immunosorbent assay (ELISA). The results obtained by Western blot analysis showed that the antiserum could react with rLacD and its mutants with highly specific and sensitive affinities.