The minimum inhibitory concentrations (MIC) for 100 strains of Streptococcus agalactiae varied as follows: ampicillin 0.1 to greater than 1 mg/l, amoxicillin 0.03 to 0.5 mg/l, cephalexin 2 to greater than 16 mg/l, nitrofurantoin 8 to greater than 64 mg/l, sulphadiazine all greater than 500 mg/l and trimethoprim less than 3.9 to greater than 250 mg/l. The MICs of the beta-lactams were not affected by inoculum density. Amoxicillin was 2.5 times as active as ampicillin. Sulphadiazine and trimethoprim acted synergistically, and the average factor of potentiation exceeded 5.9. The growth curves and drug susceptibilities of 19 strains isolated from urine were similar to those of the 81 strains isolated from other sources.