Electrophoretic variation of LDH was investigated in 3,200 specimens belonging to 28 species and 15 genera of New World monkeys. A small sample of (Old World) Cercopithecus aethiops was also tested for comparison. Variation was observed in seven species five alleles being detected for both LDHA and LDHB loci. The frequency of the variant alleles was low in almost all species, the exceptions being Callithrix kuhli and Callithrix jacchus penicillata, in which the LDHA*5 allele showed frequencies of 47% and 60%, respectively. In the monomorphic patterns the B2 and A4 bands were the same in all fifteen genera but differences were observed in the B3A1, B2A2 and B1A3 hybrid bands. Furthermore, only the B4 band was shared by humans, Old World and New World monkeys. An important marker was found in the genus Cebus, which clearly distinguishes the 'tufted' and 'untufted' groups.