alpha-Naphthylthiourea (ANTU) causes pulmonary edema and pleural effusion in rats. It has been suggested that ANTU pneumotoxicity may be mediated by blood neutrophils (PMNs) via the release of reactive oxygen species. Accordingly, we tested the effect of technical grade ANTU (tANTU) on the ability of rat peritoneal PMNs to release superoxide (O2-). tANTU did not itself stimulate O2- production by PMNs, but it increased the O2- released in response to PMN stimulation by phorbol myristate acetate (PMA). This effect was dependent upon the amount of tANTU added. In PMNs activated in vitro by a submaximal PMA stimulus, addition of 20 micrograms/ml tANTU doubled superoxide release. When tANTU was recrystallized from ethanol, the purified ANTU was not effective in potentiating the effect of PMA on PMNs. This suggests that an impurity in technical grade ANTU is capable of increasing O2- release by stimulated PMNs. tANTU and recrystallized ANTU caused similar pneumotoxicity in rats in vivo, suggesting that the unidentified impurity does not markedly influence the biologic effects of ANTU.