The thermal conductivities of blood, blood plasma, blood corpuscles, cow's milk, skimmed milk, top of milk, cream, egg white, egg yolk and cod liver oil have been measured by a comparative method. Calibration of the apparatus was performed with seven liquids, chosen for the reliability of the published values of their thermal conductivities.For the biological fluids studied, a correlation has been found between thermal conductivity and water content. Results of other workers for animal tissues are fitted approximately by this correlation, which may therefore be of some assistance in estimating the thermal conductivities of body components for which no direct determinations exist.