The effect of methantheline, a quaternary ammonium compound, on the reactivation by HI 6 of soman-inhibited human erythrocyte acetylcholinesterase (AChE) was investigated in vitro using purified human erythrocyte AChE or washed human erythrocytes. Methantheline itself was found to be a mixed competitive/non-competitive inhibitor of AChE (Kii 360 +/- 70 mumol/l; Ki 240 +/- 10 mumol/l). In all experiments the enzyme was first inhibited by soman for 30 min under conditions preventing ageing (pH 10.0 degrees C) and then ageing was allowed by changing the pH to 7.3 and the temperature to 37 degrees C. Methantheline addition (0.36 or 3.6 mmol/l) at the start of ageing increased the portion of AChE which could be reactivated by HI 6 (0.32 mmol/l) added 5 min later, from 24.6 +/- 1.0% (mean +/- SEM) of the original activity to 42.1 +/- 1.8% or 45 +/- 2.9%, respectively. With HI 6 alone, the portion of AChE activity which could be reactivated 25 min after the start of ageing decreased rapidly with the delay of the oxime addition (100% of the original activity at immediate addition), the half-life being approximately 2.5 min. With methantheline alone (0.36 mmol/l) the AChE activity was lower after immediate addition (37% of the original value), but the loss of activity due to the increasing delay of methantheline addition exhibited a similar half-life as with HI 6. Finally, when methantheline (0.36 mmol/l) was added at the start of ageing and HI 6 at various intervals thereafter the half-life of AChE activity loss due to the delay of HI 6 addition at least doubled, compared to incubations without methantheline.