Concanavalin A (Con A)-stimulated rat spleen cells were cultured in a serum-free conditioned medium. This culture supernatant contained a certain factor(s) that renders neutrophil cytotoxic for various tumor cells. The factor was tentatively termed neutrophil-activating factor (NAF). Rat NAF was partially purified from the serum-free culture supernatant by using ion exchange chromatography of DEAE-Sephadex A-50, gel filtration of Sephadex G-100, and affinity chromatography of Con A-Sepharose 4B. NAF activity was eluted in broad fractions by the ion exchange chromatography and the gel filtration. Moreover, on the Con A column, some NAF activities were bound to the column, but other activities passed through the column. These results showed the heterogeneity or polydispersity of NAF activity in both molecular size and charge-based separation properties. Monoclonal antibodies were produced by fusing BALB/c myeloma cells (P3-X63 Ag8.653) with spleen cells from syngeneic mice immunized with partially purified NAF (pNAF) obtained from the gel filtration. Absorbent beads which were linked with one monoclonal antibody (ANAF-10) partially absorbed NAF activity from supernatants of a Con A-stimulated spleen cell culture. Further purification of pNAF was performed with the use of affinity chromatography of ANAF-10-linked Sepharose. Through these procedures, the NAF activity was concentrated about 10,000-fold. Heterogeneity of NAF activity, however, did not disappear in even this affinity chromatography. On the other hand, 125I-labeled material of the final product migrated to one major band corresponding with an m.w. of about 20,000 as determined by SDS-PAGE analysis, and NAF activity was detected in the same band.