The dermatophyte fungus Trichophyton rubrum often produces arthroconidia in vivo, and these cells are thought to be involved in pathogenesis, and, in shed skin scales, to act as a source of infection. The purpose of this study was (i) to examine the environmental and iatrogenic factors which affect arthroconidiation in T. rubrum in vitro, (ii) to look at arthroconidia formation in a large number of clinical isolates of T. rubrum and (iii) to develop a new model for the study of arthroconidia formation in nail tissue. Arthroconidia production was studied in T. rubrum grown on a number of media and under conditions of varying pH, temperature and CO(2) concentration. The effect of the presence of antifungals and steroids on arthroconidia formation was also examined. Nail powder from the healthy toenails of volunteers was used as a substrate for arthroconidial production. On Sabouraud dextrose agar in the presence of 10 % CO(2) plus air, arthroconidial formation occurred optimally at 37 degrees C and pH 7.5, and was maximal at 10 days. Most isolates of T. rubrum showed a similar level of arthroconidial production, and only two out of 50 strains were unable to produce arthroconidia. Subinhibitory levels of some antifungals and betamethasone resulted in the stimulation of arthroconidia formation. Arthroconidial production in ground nail material also occurred under the same optimal conditions, but took longer to reach maximal levels (14 days). These in vitro and ex vivo results provide a useful basis for the understanding of arthroconidium formation in vivo in infected tissues such as nails.