Summary A method of protoplast isolation and culture from in vitro grown haploid Nicotiana sylvestris plants has been developed for application to mutant research. The influence of plant growth conditions on protoplast yield has been stressed. The composition of the plant culture medium has been shown to have a stricking effect on protoplast yield and ability to divide. The best medium was a modified Skoog with low (10 g/1) sucrose and auxin (NAA 10-8), but normal vitamin concentration. A low salt medium devoid of vitamins and auxin gave a very poor yield of protoplasts unable to divide. The other plant growth condition studied was light intensity. The optimum was found at 5000 Lux. A medium for protoplast culture was developed from the Tt medium of BOURGIN et al. (1976) by increasing the vitamin concentration a 2.5 fold and adding 2 0/o conditioned medium. In that medium, protoplasts from in vitro grown haploids divided readily and formed colonies at a 100 °/o plating efficiency. A dramatic effect of initial protoplast density on plating efficiency has been demonstrated. The optimum range was 3000 to 10,000 protoplasts/ml. Out of this range, the plating efficiency droped very fast. Viable protoplasts were regularly isolated and high plating efficiency was very reproducible. No contaminations have been encountered in 20 experiments. These features render the method very reliable and well suited to mutation studies. In order to determine a mutagenic treatment, the effect of increasing doses of gamma rays from Co on plating efficiency was studied. The response followed the classical shoulder curve. The 50 0/o lethal dose was about 1000 rad for haploid protoplasts.