Protection of mice from fatal neuroadapted Sindbis virus encephalitis can be accomplished by passive transfer of monoclonal antibodies (MAbs) to either the E1 or E2 glycoprotein of Sindbis virus. Both neutralizing and non-neutralizing MAbs can be protective. To define further the characteristics of MAbs that provide protection from fatal disease, antigenic epitopes on the E1 and E2 glycoproteins were identified using a competitive binding enzyme immunoassay. Four distinct epitopes on E1 and three on E2 were defined. MAbs to all E1 epitopes, both neutralizing (three) and non-neutralizing (one) protected mice from fatal encephalitis. MAbs to the E2 neutralizing epitopes (two) protected mice from fatal encephalitis while those to the non-neutralizing epitope did not. The efficiency of protection from fatal Sindbis virus encephalitis of four neutralizing and non-neutralizing protective anti-E1 and anti-E2 MAbs representing different epitopes was compared. The neutralizing MAbs (against epitopes E2-ab, E2-c and E1-c) gave 50% protection at lower doses (2 to 20 micrograms) than the non-neutralizing MAb representing epitope E1-e (150 micrograms) when given before virus challenge. When given after virus challenge, MAbs to E2-ab and E2-c protected at lower doses (0.03 to 0.3 micrograms) than did either MAbs to E1-c (greater than 100 micrograms) or E1-e (10 micrograms). The MAbs to E1-e, E2-ab and E2-c were required in larger amounts to afford protection before than after challenge, while the opposite was true for MAb to E1-c.