Abstract Using the fluorescent probe DPH (1,6-diphenyl-1,3,5-hexatriene), steady-state anisotropy ( r s) values were determined in red blood cell (RBC) ghost membranes of schizophrenic patients before ( n=26) and after ( n=16) haloperidol withdrawal, and age- and sex-matched normal controls ( n=22). Patients on haloperidol were treated with doses between 5 and 20 mg/day for at least 5 weeks. Drug-free patients had been free of all psychotropic medication for an average of 36 days. The mean r s value of haloperidol-treated patients (0.2422−0.0076) was not significantly different from that of either the drug-free patients (0.2445−0.0092) or the normal controls (0.2383−0.0083). A significant increase ( p=0.037) in r s values, however, was observed in the drug-free schizophrenic patients as compared to that of the controls. Furthermore, the mean r s values of relapsed drug-free patients were significantly ( p<0.05) higher than in the same individuals on haloperidol treatment. Changes in r s values of drug-free patients were also significantly correlated with their increase in psychosis ratings ( r=0.74, p=0.001). The ‘structural order’ of RBC ghost membrane, however, was not correlated with age, age of onset, daily haloperidol dose, plasma haloperidol levels, plasma cholesterol concentrations, or the duration of illness, haloperidol treatment or drug-free period. Thus, the present data demonstrate that RBC membrane fluidity is not affected by haloperidol treatment. Following haloperidol withdrawal, however, those patients with decreased RBC membrane fluidity, which correlated with increased psychosis, may be prone to relapse within 6 weeks.