natural killer activity against human k562 tumour cells during plasmodium cynomolgi malarial infection of rhesus monkeys this study assessed the natural killer nk cell activity profile during plasmodium cynomolgi infection in rhesus monkeys there was a significant decrease in the nk cell activity in the peripheral blood leukocytes of infected monkeys during the early ascending phase of infection however as the parasite load decreased nk cell activity returned to normal levels this could be correlated with the peak increase in lymphocyte counts this indicated that a decrease in nk cell activity observed at an earlier stage during an active p vivax malarial infection was a temporary phenomenon