We have previously isolated 3T3 cell variants unable to respond to specific mitogens. In this report we analyze the dominant and/or recessive nature of these variants. (1) Two independently isolated EGF nonproliferative variants are unable to bind EGF. Hybrids between 3T3R5 cells (thymidine kinase deficient, ouabain-resistant) and these variants express EGF receptors; the “EGF receptorless” phenotype of these variants is recessive. (2) Hybrids between these two variants do not bind EGF; they are defective in a common, non-complementing function. (3) A TPA nonproliferative 3T3 variant is also recessive; hybrids with 3T3R5 mount a mitogenic response to TPA. (4) In contrast a fourth variant, which can neither bind labeled EGF nor respond to TPA, is dominant for both characteristics. Hybrids between this latter variant and 3T3R5 can neither bind EGF nor mount a mitogenic response to TPA.