Investigations were carried out to assess the effects of disodium 1-hydroxyethane-1,1-diphosphonate and disodium dichloromethanediphosphonate (compounds containing a P-C-P bond) on isolated rabbit articular chondrocytes in culture. Studies on growth behaviour showed that both diphosphonates displayed inhibitory actions, dichloromethanediphosphonate producing the larger effect. Both compounds inhibited the uptake of 2-deoxy-d-glucose, dichloromethanediphosphonate once more being the more potent of the two. The uptake of alpha-aminoisobutyrate was considerably increased by chondrocytes treated with dichloromethanediphosphonate, whereas 1-hydroxyethane-1,1-diphosphonate showed no effects. The biosynthesis of sulphated extracellular macromolecules secreted by the cells into the pericellular space as well as into the growth medium was greatly increased by dichloromethanediphosphonate but not by 1-hydroxyethane-1,1-diphosphonate. The stimulatory effect was dose-dependent. Short-term exposure of already confluent cells to dichloromethanediphosphonate as opposed to growing the cells to confluence in the presence of the diphosphonate revealed that the stimulatory effects were already evident after 24h, indicating that cell division is not necessarily required to produce the observed effects. The increment in proteoglycan synthesis was still evident with cells that were exposed continuously to the diphosphonate in primary as well as secondary culture. Pulse-chase experiments together with studies on the enzyme arylsulphatase revealed that the appearance of increased amounts of proteoglycans was the result of a stimulation in synthesis and not due to an inhibition in turnover.