Minimum inhibitory concentrations (MIC) for penicillin (PEN), ampicillin (AMP), cephalothin (CEF), cefazolin (CEZ), carbenicillin (CAR) and ticarcillin (TIC) were determined against Strep. faecalis, E. coli, K. pneumoniae, Prot. mirabilis and Ps. aeruginosa in Sensitivity Test Broth (STB) and human urine. Marked differences were seen in MIC between these two media, especially against P. mirabilis strains tested. Against E. coli the MIC of AMP, CEF and CEZ were similar or slightly lower in urine than in STB. MIC for CAR against E. coli were higher in urine. Against K. pneumoniae the MIC for CEF and CEZ were 2-4fold higher in urine than in STB. The activity of CAR and TIC against Ps. aeruginosa was lower in urine. Albeit high concentrations of antibiotics (up to 800fold) no inhibition was achieved in urine against P. mirabilis by AMP, CEF or CEZ. Only against Strep. faecalis all tested antibiotics had lower MIC in urine than in STB.