The stability of ceftriaxone in undiluted human serum and in human serum diluted 1/20 in 0.1 M phosphate buffer (pH 6.0) was measured at -70, -40, -20, 4, and 37 degrees C. Ceftriaxone in diluted human serum contained at least 90% of the initial activity after 3 months of storage at -20, -40, and -70 degrees C; unbuffered human serum contained 82.41, 84.92, and 88.96% of the initial activity, respectively. Ceftriaxone in unbuffered human serum and in diluted human serum showed 80.33 and 86.25% of the initial activity, respectively, after 55 days at 4 degrees C. After 120 h at 37 degrees C, this antibiotic in unbuffered human serum contained 33.08% of the initial activity, whereas samples of diluted human serum contained more than 60%. Consequently, the stability of antibiotic in human serum diluted 1/20 in 0.1 M phosphate buffer (pH 6.0) is increased.