The monocyte chemotactic activity elaborated by soluble dental plaque material and by various fractions of the plaque material was studiedin vitrowith the modified Boyden chamber technique. Monocytes were harvested from the peritoneal cavity of rabbits after intraperitoneal injection of a 1 % suspension of hydrolysed starch. The plaque material was separated in Sephadex gel columns (G‐25 and G‐20Q). Three fractions differing from each other in molecular size were selected from the protein peaks in the chromatograms.The results showed that dental plaque material contains factors with a chemotactic effect on monocytes. Examination of various fractions of dental plaque material revealed that the major part of the monocyte chemotactic activity resided in a fraction with an appoximate molecular size of 10,000–7