The effect of antacid on the bioavailability of cefprozil was investigated in a two-way crossover study. Eight healthy male subjects received a single 500-mg oral dose of cefprozil with and without coadministration of 30 ml of an antacid suspension containing magnesium hydroxide and aluminum hydroxide (Maalox). Cefprozil consists of cis and trans isomers in an approximate 90:10 ratio. When cefprozil was administered alone (treatment A), the mean maximum concentrations (C..,) of the cis and trans isomers were 9.2 and 1.2 ag/ml, respectively. When cefprozil was coadministered with Maalox (treatment B), the Cm.x values of the cis and trans isomers were 8.7 and 1.3 ,g/ml, respectively. The mean values of the area under the curve from time zero to infinity (AUC>.C) were 27.7 and 3.5 ag-h/ml for treatment A and 27.5 and 3.5 pg h/ml for treatment B for the cis and trans isomers, respectively.