We have examined the effects of commercially available preparations and drug-free solvents of diazepam (Valium, Diazepam-Lipuro) and midazolam (Dormicum) by N-formylmethionyl-leucylphenylalanine (FMLP)- and zymosan-induced polymorphonuclear cell (PMN) chemiluminescence and in a cell-free chemiluminescence system. In the case of Valium, drug-free solvent and diazepam suppressed PMN chemiluminescence. With Diazepam-Lipuro, the solvent stimulated and diazepam inhibited PMN chemiluminescence. With midazolam (Dormicum), only the active drug depressed PMN chemiluminescence. FMLP-induced PMN chemiluminescence was depressed 10-100 fold more by diazepam and midazolam than zymosan-induced chemiluminescence.