Nuclear refringence is a phenomenon visible by light microscopy that is correlated with lymphocyte maturation and activation. The present study, in employing isolated nuclei as well as intact lymphocytes, confirms the nuclear basis of the phenomenon. Enzymatic degradation of nucleic acids in isolated nuclei attenuates refringence, providing presumptive evidence for the involvement of chromatin in refringence. Cytoplasmic events involving RNA and protein synthesis and microtubular, but not microfilament, function are necessary for the maintenance of refringence. These studies provide the basis for evaluating refringence within the context of nuclear protein changes known to occur during lymphocyte activation and for determining its usefulness as a simple test of the dynamic state of the lymphocyte nucleus.