A semi-automatic micromethod was developed using 96-well microtiter plates in combination with a video monitoring system to assay human tumor clonogenic growth in vitro. The conditions to follow the growth of K562 human myeloid colonies were optimized and validated: About 250 cells in a 40 mul agar mixture are to be seeded per well. The colony number is proportional to the number of seeded cells. The cloning efficiency is about 75% and correlates well with the established glass capillary method. The system precision of the automatic colony counter, expressed by the coefficient of variation, is less than 2%. The mean of the intra- and inter assay coefficient of variation is low (8.5% and 7%, respectively). The microassay was applied to measure the lymphokine-activated killer (LAK)-activity of peripheral blood mononuclear cells (PBMC) from healthy donors against K562 target cells and to examine the effects of various thymic hormone preparations on the cytotoxicity of generated LAK cells. Thus, the new microassay provides a useful tool for measuring clonogenic tumor cell growth and its modulation by different agents on many samples in a short time.

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