In summary, the results of this study are indicative that antibiotics can modify the synthesis of several prostanoids. A mixture of antimicrobials that is commonly used in culture media caused a decrease in the production of PGE2 and TXB2 (but not PGF(2α)). Testing of the individual component drugs produced results suggestive that the inhibitory activity resides in the streptomycin, penicillin and amphotericin being without effect in the concentrations tested. The prostaglandin synthase inhibitory effect of the streptomycin at concentrations used in tissue cultures is probably not of sufficient magnitude by itself to alter significantly prostaglandin production rates. It has been reported, however, that aminoglycoside antibiotics can also inhibit phospholipase C activity.