Temporal variations in the expression of phosphoprotein phosphatase 1 (PP1), phosphoprotein phosphatase 2A (PP2A) and protein tyrosine phosphatase 1B (PTP1B) were monitored in the human acute, promyelocytic leukaemia cell line, HL60. Granulocytic differentiation was induced using all‐trans retinoic acid (ATRA) and monocytic differentiation by phorbol‐12‐myristate‐13‐acetate (PMA). Expression of the enzyme proteins in cell extracts was determined by SDS‐PAGE and Western immunoblotting using specific antibodies. For PP1, a single immunospecific band of molecular mass 38kDa was detected corresponding to the catalytic subunit; induction of differentiation with either ATRA or PMA showed differences in the patterns of expression and, in the case of the latter, the mean value. Two immunospecific bands, of mass 34 and 37kDa, possibly corresponding to dephosphorylated and phosphorylated forms, respectively, were detected for PP2A, as well as a minor band of mass 46kDa; dynamic variations in the expression of all 3 forms were observed and there were differences between the control and treated cells. The catalytic domain of PTP1B was detected as a 46kDa band. A 42kDa form of the protein was also seen, which may represent a change in phosphorylation state, or be the result of proteolytic cleavage; usually the 46kDa band was the major form, but on occasion there was a change to predominance of the 42kDa band.