Two experiments were conducted to evaluate developmental gene expression of antimicrobial peptide Protegrin-1 and effect of weaning on gene regulation of Protegrin-1 in piglets using semi-quantitative RT-PCR analysis. In experiment 1, total 15 female 28 day-age weanling Duroc × Landrace×Yorkshire piglets were divided into three groups. And 5 piglets in each group were slaughtered at 1, 14, 28, 42 and 56 day-age respectively, which were used to determine mRNA expression of Protegrin-1 at different ages. In experiment 2, 18 neonatal female piglets, divided into three groups (6 piglets of each group), were weaned at 21, 28 and 35 days of age, respectively. At the 21, 28 and 35 day-age before weaned, 3 piglets in each group were slaughtered, and bone marrow cells of piglets were sampled. Furthermore, additional 3 piglets in each group were slaughtered and sampling in 7 days after they were weaned. The samples were collected for detecting the effect of weaning on Protegrin-1 mRNA expression of piglets. The current results show that Protegrin-1 mRNA levels increased steadily in postnatal 1-28 day-age (pre-weaning). But after the piglets were weaned at 28 day-age, the levels of Protegrin-1 gene expression were decreased markedly (P<0.05) from 28 to 42 day-age (14 days after weaning). From 42 days to 56 days, its expression levels were increased again. The results of experiment 2 showed that when the piglets were weaned at 21 days of age, the expression levels of Protegrin-1 were decreased by 34% at the seventh day after weaning; when the piglets were weaned at 28 days of age, the expression levels of Protegrin-1 were decreased by 51% at the seventh day after weaning; when the piglets were weaned at 35 days of age, the expression levels of Protegrin-1 were decreased by 49% at the seventh day after weaning. Our results showed that weaning significantly decreased Protegrin-1 mRNA expression of piglets.