Dr. Ericsson also said his method was unlikely to alter the sex balance of the population. He said couples who use his method are about evenly divided, with 52 per cent favoring girls. In most cases, it is parents who have had children of only one sex would like one child of the other. Artificiality Poses Barrier

Another brake on widespread use of the method is its artificiality. It requires a number of steps - semen collection, multi-step sperm segregation, and artificial insemination. The American Fertility Society noted in a report last year on ethical considerations of new reproductive technologies that artificial insemination ''may make reproduction excessively technologic by separating procreation from sexual expression.''

Dr. Ericsson scoffed at the suggestion that the procedure raises moral problems. He contrasted this with cases reported from India where a fetus was allegedly aborted if, from tests in early pregnancy, it was not of the desired sex. Such ethical questions are on the agenda of the World Congress on Human Reproduction to be held in Tokyo next month with Dr. Iizuka as president. Based on Density Differences

Dr. Iizuka's said his procedure was a variation of a method used to sort out blood cells of slightly different composition. It is based on differences between sperm carrying X chromosomes, which produce girls, and Y chromosomes. The DNA content of X-bearing sperm has been found 2.7 percent greater than that of Y-bearing sperm.

The rate at which the cells migrate through a silica gel, when subjected to rapid spinning in a centrifuge, depends on their size, mass or other subtle properties.

In the sex-selection method, a layer of diluted semen is laid on top of a sequence of increasingly dense layers of a Swedish silica gel, Percoll, and for 20 minutes of centrifugation is subjected to a force 250 times that of gravity. Sperm containing the female chromosome migrate through the layers slightly faster than those with the male chromosome.

Male sperm can be identified because a spot within their nuclei glows when stained with a fluorescing compound. After centrifugation, Dr. Iizuka said, 85 percent of the sperm in the top layer proved to be male and 95 percent of those at the bottom were female. Sperm drawn off from each layer could then be used for insemination.