This report describes a noninvasive 1H-NMR method for measuring absolute temperatures (+/- 0.2 degrees C) in biological samples and, in particular, in cell culture perfusion systems, utilizing the linear temperature dependence of the water chemical shift relative to the temperature-independent shift of one of the components of the biological medium, e.g., pyruvate, acetate or lactate. The effects of flow on temperature can be monitored and appropriate adjustment of the temperature controller can be made.