Sponges (Porifera) represent the lowest metazoan phylum; they have been shown to be provided with the characteristic metazoan structural and functional molecules. One autapomorphic character of sponges is the presence of high levels of telomerase activity in all cells (or almost all cells, including somatic cells). In spite of this fact previous attempts to cultivate sponge cells remained unsuccessful. It was found that dissociated sponge cells do not replicate DNA and lose their telomerase activity. In addition, no nutrients or metabolites have been detected that would stimulate sponge cells to divide. In the present study we report the culture conditions required for the formation of multicellular aggregates from dissociated single cells of Suberites domuncula, termed primmorphs. These primmorphs are formed in seawater without addition of further supplements, and have an organised tissue-like structure; they have been cultured for more than 5 months. Cross-sections revealed a distinct external layer covered by a continuous pinacoderm, and a central zone composed primarily of spherulous cells. After their association into primmorphs, the cells turn from the telomerase-negative state into the telomerase-positive state; a telomerase level of 4.7 total product generated (TPG) units/5 x 10(3) cell equivalents has been determined. Moreover, a major fraction of the cells in the primmorphs undergoes DNA synthesis and hence has the capacity to grow. Applying the BrdU-labelling and detection assay it is demonstrated that up to 33.8% of the cells in the primmorphs are labelled with BrdU after an incubation period of 12 h. It is proposed that the primmorph system described here is a powerful novel model system to study basic mechanisms of cell proliferation and cell interaction, as well as of morphogenesis, ageing and apoptosis.