We have previously shown that in several strains of mice morphine abolishes inflammation-related pain symptoms already at 5 mg/kg, but inhibits inflammation only at 20 mg/kg. Other investigators have shown that also riboflavin (vitamin B2) has antinociceptive and anti-inflammatory activities. In particular, highly purified riboflavin is a promising therapeutic agent for both bacterial infections and toxin-induced septic shock, mainly due to inhibition of pro-inflammatory cytokines/chemokines and nitric oxide production. The aim of present study was to examine anti-inflammatory effects of morphine, riboflavin and combined action of the both agents injected intraperitoneally (i.p.) 30 min before zymosan treatment in Swiss mice. Animals pre-injected with PBS formed the control group. At 4th hour after PBS-Zymosan injection, the number of peritoneal PMNs reached 20 × 106. The number of PMNs was similar in animals pre-injected with low dose of morphine (5 mg/kg b.w.) or riboflavin (20 and 50 mg/kg b.w.) while intraperitoneal influx of PMNs was significantly inhibited in animals pre-injected with high dose of morphine (20 mg/kg b.w.) or riboflavin (100 mg/kg b.w.). Similar anti-inflammatory effect was achieved by pre-injection with the combined low dose of morphine (5 mg/kg b.w.) with riboflavin (50 mg/kg b.w.).