The present study was planned to study the effect of priming of BALB/c mice peritoneal polymorphs (PMNLs) by viable, heat killed and formaline fixed promastigotes of L. donovani and soluble extract of proteins of L. donovani promastigotes on their capacity to produce toxic oxygen radicals (TOR). TOR production was measured by superoxide dismutase (SOD), inhibitable cytochrome C reduction and nitroblue tetrazolium reduction tests. No significant stimulation or inhibition in SOD inhibitable cytochrome C reduction and nitroblue tetrazolium reduction was seen when PMNLs were primed by soluble leishmanial protein, and formalin fixed and denatured promastigotes for both 60 and 90 min. Priming of BALB/c mice peritoneal polymorphs with live promastigotes of L. donovani for more than 60 min produced approximately 30 per cent inhibition in superoxide generation as measured by both the tests (P < 0.05). The inhibition in superoxide generation by polymorphs may be one of the important escape mechanisms in the causation of the visceral leishmaniasis.