Monoclonal antibodies (MAbs) have the potential for diagnosis and therapy of cancer. 5G6.4 is a MAb of the IgG2a class which was produced by immunization of BALB/c mice with human ovarian carcinoma (Ov Ca) cells. To further characterize 5G6.4, its effect on cell growth was tested using a human Ov Ca cell line established in our laboratory. A clonogenic assay was set up in 48-well plates in a double agar system. The cells were plated and 5G6.4 was added at different concentrations. Control plates consisted of cells with media without MAb. Negative control plates were also prepared using the same concentrations of an isotype-matched antimelanoma MAb, 225.28s. Colony formation (CF) was reduced to 50% or less of control with increasing amounts of 5G6.4 up to 50 μg/ml. Although CF was still depressed at concentrations above 50 μg/ml, the inhibition did not follow a directly proportional line; instead, it followed a bell-shaped curve. Plates with the control MAb, 225.28s, did not show this response. Similar results were obtained with cells from malignant Ov Ca ascites in the same clonogenic assay. Our study suggests that in the evaluation of the in vitro effect of MAb on growth, the concentration of MAb is crucial and may not show a linear response and that 5G6.4 may have a direct therapeutic effect by blocking the growth of Ov Ca cells. 5G6.4 is presently under study for therapy in an animal model.