To estimate the minimum dose required to infect a monkey, rhesus monkeys were inoculated with LACV/mosquito/1978-cl at 10 1 or 10 3 PFU subcutaneously. Blood was collected on days 0, 21, 28, and 42, and serum neutralizing antibody titers were determined. Mean neutralizing antibody titers were 1:355 and 1:82 for the 10 1 or 10 3 PFU dose groups, respectively, and all monkeys seroconverted by day 28 (PRNT 60 ≥ 40) (Table 4 ). Thus, LACV is highly infectious for rhesus monkeys even at a dose of 10 1 PFU and results in the induction of a high level of neutralizing antibody. However, LACV infection did not result in identifiable clinical abnormalities in this group of 24 monkeys. 