Scream vocalizations produced by pigtail macaques during agonistic encounters were classified according to caller body weight (small, medium, large) on the basis of six frequency-related acoustic variables using direct discriminant analysis. Separate discriminant analyses were run for: 1. calls produced when the attack involved an opponent higher-ranking than the victim and contact occurred, and 2. calls produced when the opponent was higher-ranking, but no contact occurred. Screams were correctly classified as to the caller's weight class at levels significantly above those expected by chance alone. Screams given during contact aggression were classified significantly better than those given in the absence of contact. The effect of greater arousal level (fear) on the frequency range of calls may account for this difference. Macaque screams are representational signals that are important in the solicitation of agonistic aid from allies in the social group. That a relationship between body size and the frequency of screams is nonetheless evident argues for the fundamental nature of the relationship.