The objective of this study was to investigate the therapeutic effects of inactivated Propionibacterium acnes and lipopolysaccharide derived from Escherichia coli cells in cats affected by feline panleukopenia virus (FPV). A retrospective study of 80 FPV-positive cats was divided into two groups: a treatment group receiving inactivated Propionibacterium acnes and lipopolysaccharide derived from Escherichia coli cells along with supportive treatment and a no-treatment group receiving only supportive treatment. There was no significant difference in the total white blood cell counts between the two groups. However, the total white blood cell counts of both groups were low on day 0 and increased significantly on days 3 and 6 of treatment. Additionally, the white blood cell counts in the treatment group significantly increased during days 3 to 6 compared with those of the no-treatment group (p < 0.01). The mortality rate was not significantly different between the two groups. In a prospective study, the serum and fecal immunoglobulin A (IgA) levels were measured in both groups. There were no significant differences in IgA levels between the two groups in either the serum or feces.