The effect of a pasture soil suppressive to Pythium splendens in the South Kohala district on the island of Hawaii on germination or growth of various microorganisms ranged from inhibitory to stimulatory compared with that of conducive soil from Hilo. The soil was suppressive to damping-off caused by Pythium splendens and Pythium aphanidermatum, but not to damping-off caused by Phytophthora palmivora and Phytophthora capsici and it inhibited germination of spores of Pythium splendens and Pythium aphanidermatum, but not those of Phytophthora palmivora and Phytophthora capsici. Sterilization by autoclaving or .gamma.-irradiation completely nullified the inhibitory effect of suppressive soil. Inhibition of germination of sporangia of Pythium splendens was restored to sterilized suppressive soil by reinfestation by adding suspensions of either suppressive or conducive soil. Reinfestation of sterilized conducive soil with suppressive soil suspension did not convert it to inhibitory soil. Inhibition of sporangia germination also was restored in autoclaved suppressive soil by reinfestation with actinomycetes, bacteria or fungi. In mixtures of suppressive and conducive soil, the inhibitory effect of suppressive soil varied inversely with the proportion of conducive soil. Germination inhibition of mixed soil decreased after incubation for 14 and 28 days. Amendment of suppressive soil with 333 ppm of rose bengal, 6667 ppm of streptomycin, or 3333 ppm of benomyl partially nullified its inhibition of sporangium germination. The solution obtained from suppressive soil amended with rose bengal or streptomycin was inhibitory to bacteria and actinomycetes but not fungi, whereas the solution obtained from benomyl-amended soil was inhibitory to fungi, but not bacteria and actinomycetes. Neither volatile nor nonvolatile inhibitors were detected in suppressive soil. Suppressive soil remained inhibitory to germination when its pH was adjusted from 6.8 to 5.4, and conducive soil remained conducive to germination when its pH was adjusted from 5.4 to 6.8. A combination of unspecific microbiological factors and unknown abiotic factors is apparently responsible for the microbiological suppressiveness of this pasture soil from South Kohala.