Abstract: BACKGROUND ＆ AIM: To explore an economic,quick and valid method for counting foci in vitro micromass cell cultures,and to provide a useful procedure relevant to this short-term test. MATERIAL AND METHODS: Midbrain cells and limb bud cells were collected from pregnant female rats at 13 days after conception and cultured constantly for 5 days in culture medium.The foci was counted by the new method introduced:the light microscopy field was divided into several compartments, moving across the compartments regularly and counting the numbers of foci.This paper also evaluated the accuracy and sensitivity of this procedure by testing reproducibility. RESULTS: The validity of test data was obtained via Grubbs test.The relative error of midbrain and limb bud cells were 0.776％ and 0.213％;systerm standard deviations were 0.933 9 and 2.883 9; the coefficients of variation(CV) were 1.24％ and 2.05％, respectively. CONCLUSION: High accuracy and seusitivity was obtained by the use of new parallel section mensuration method.The procedure could be used as aroutine method in micromass embryo cell cultures.

Key words: micromass culture, midbrain cell, limb bud cell, mensuration method