ABSTRACT: In riverine water, both suspended particulate material and viruses are prominent ecological factors. The existence of various particle types and differences in viral abundance impose variability in microenvironments. Particulates and their microbial surrounding may interact in several ways, this interaction being strongly dependent on particle quality and the abundance of organisms involved. In laboratory experiments, we used different suspended matter types (fresh and aged mineral sediment and leaf litter, river snow) that typically occur in riverine environments as model particles. We investigated the effects of particle quality and different ambient viral abundances (×1, ×2 enrichments, and inactivated viruses) on several microbial parameters (changes in bacterial and viral abundances, bacterial production, specific bacterial production) of both the free-living and particle-attached fractions using water from a floodplain system of the Danube River (Austria). Both seston quality and variable viral abundances in the bulk water influenced some microbial parameters. The average abundance of bacteria and viruses was significantly higher on organic than on inorganic particles and on aged particles (for both sediment and leaf litter). Changes in bacterial abundance during the course of the experiments were also influenced by particle quality, with, for example, aged sediment favoring increasing abundances. Virus:bacterium ratios (VBR) were significantly higher on organic than on inorganic particles, but significantly lower on suspended particles than in the planktonic fraction. Typically, bacterial secondary production (overall and cell-specific) was higher on particles than in bulk water. Bacterial productivity in the ambient water was negatively affected by the abundance of planktonic viruses but positively affected by that of attached viruses. These findings from experimental systems may foster in situ studies of particle-rich environments.