Poly(N,N-diethylacrylamide) microgels (diameter < 1000 nm) with narrow size distribution were synthesised by Surfactant-Free Emulsion Polymerisation at 70 degrees C in aqueous solution using potassium persulphate as initiator. The gels were characterised in comparison to the corresponding poly(N-isopropylacrylamide) microgels. They showed reversible collapse upon thermo-stimulation with a critical temperature between 28 and 30 degrees C, i.e. slightly below the critical temperature of the corresponding linear molecules. The average particle diameter as determined by electron microscopy decreased with increasing stirring rate adjusted during synthesis, although the effect was less straightforward for the poly(N,N-diethylacrylamide) microgels than for the poly(N-isopropylacrylamide) ones. In microgel preparations produced at the highest stirring speeds, the presence of small particles was observed, possibly a manifestation of residual 'precursor particles'. A first characterisation of the biocompatibility of the gels was done by the WST-1 viability and the LDH cytotoxicity assays using the human breast cancer cell line MCF-7 (ATCC HTB-22) and a human T leukaemia cell line (Jurkat, ATCC TIB-125) as probes. No short-term effects (time span covered in the investigation <4 h) of the microgels on cell viability could be observed when the cell were incubated with the gels at 37 degrees C, i.e. above the critical temperature. At 22 degrees C and for microgel concentrations above 1 mg/mL some cytotoxicity was observed. In general the cytotoxicity was more pronounced for the poly(N-isopropylacrylamide) microgels than for the poly(N,N-diethylacrylamide) ones. (c) 2006 Elsevier Ltd. All rights reserved.