We have measured the ultrasonic attenuation parameters α0 and n of several biological fluids in vitro in the frequency range 2.8–6.8 MHz, using frequency shift and/or a sinc (⋅) sidelobe ratio technique. The parameters α0 and n describe the frequency-dependent attenuation via a power-law model, i.e., α( f)=α0 f n, where f is frequency. The samples investigated were blood, pus, cyst fluid, bile, and infected hematoma. It was found that the values of α0 span approximately the range between 0.03–0.3 dB cm−1 MHz−n, and values of n range from about 1.1–1.3.