Cellular oscillations in total extractable protein content and protein tyrosine phosphatase activity were analysed in proliferating and differentiating human acute promyelocytic leukaemic (HL-60) cell extracts. Differentiation was induced along the granulocytic pathway using all-trans retinoic acid (ATRA). High frequency rhythms with distinct, well defined waveforms of varying amplitude were observed for both the total protein content and for the enzyme activity of protein tyrosine phosphatase (PTP). Linear correlation analysis showed that there was no obvious relationship between the protein content and the corresponding PTP activity, suggesting that the periodic variations between these two components are relatively independent of each other. ATRA significantly altered the characteristics of the rhythms with respect to the period and amplitude and had a dampening effect on the oscillatory pattern of PTP activity. Modulation of such characteristics may be of significance with respect to the regulation of the differentiation processes and the possible reversal of transformation.