To measure possible changes in basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from insulin-resistant individuals, soluble and particulate muscle fractions were prepared from biopsies taken before and after a 3-h hyperinsulinaemic euglycaemic clamp in eight non-insulin-dependent diabetic (NIDDM) patients and nine control subjects. We used a sensitive sandwich-immunofluorescence assay and the human insulin receptor as the substrate. PTPase activity was expressed as percentage of dephosphorylation of phosphotyrosyl-residues in immobilized insulin receptors per 2 h incubation time per 83 micrograms and 19 micrograms muscle fraction protein (soluble and particulate fraction, respectively). In the diabetic soluble muscle fractions, the basal PTPase activity was decreased compared with that of control subjects (11.5 +/- 5.5 vs 27.5 +/- 3.3, p < 0.04, mean +/- SEM). In the particulate muscle fractions from the control subjects, PTPase activity was increased after 3 h hyperinsulinaemia (20.0 +/- 3.2 vs 30.2 +/- 3.6, p < 0.03) and in the corresponding soluble fractions PTPase activity seemed decreased (27.5 +/- 3.3 vs 19.9 +/- 5.9, NS). No effect of insulin on PTPase activity was found in NIDDM patients (25.1 +/- 4.1 vs 27.2 +/- 5.2, 11.5 +/- 5.5 vs 15.1 +/- 4.5 [particulate and soluble fractions], NS). In conclusion, we found that the basal PTPase activity in soluble muscle fractions was decreased in NIDDM patients; furthermore, insulin stimulation was unable to increase PTPase activities in the particulate fractions, as opposed to the effect of insulin in control subjects.