During pot trials, germination of barley and ryegrass seeds appeared to be inhibited by the presence of sewage sludge. Laboratory experiments confirmed this effect and showed that germination was not permanently inhibited but merely delayed. The lag period was dose-dependent, increasing in proportion to the amount of sludge added. A similar effect was induced by heavy metals (Cu, Ni, Zn) in aqueous solution but only by concentrations much higher than those in sludge, considering that most of the total metal load in sludge is in insoluble or unavailable forms. Results showed that the effect was more closely related to the organic matter (OM) content of sludge-soil mixtures on which the seeds were incubated. When sets of old and new mixtures differing only in OM content were tested, the delay in germination was greatest in the new mixtures which contained higher amounts of organic matter. When sludge is freshly incorporated with soil, intense microbial activity is to be expected leading to reduced oxygen tensions and conditions which favour the formation of volatile inhibitors such as ethylene and ammonia. Preliminary tests established that volatile inhibitors were involved in the effect.