The invention relates to compounds of the formula I, in which R1 and R2 are, independently of each other, lower alkyl or lower alkoxy-lower alkyl; R3 and R4 are, independently of each other, sec-lower alkyl or tert-lower alkyl; R5 is phenyl or cyclohexyl; and R6 and R7 are, independently of each other, lower alkyl, or, together with the linking nitrogen atom, pyrrolidino, piperidino, 4-lower alkylpiperidino, 1,2,4-triazol-1-yl or 1,2,4-triazol-4-yl; or a salt thereof, provided that at least one salt-forming group is present. These compounds are inhibitors of HIV protease and are therefore suitable, for example, for treating AIDS or its preliminary stages.