Replacement of leucine 301 in the human colony stimulating factor 1 receptor (CSF-1R) by serine, threonine, glutamic acid, or proline induced ligand-independent transforming activity in mouse NIH3T3 cells, whereas substitution by phenylalanine, methionine, cysteine, or lysine did not. Serine, glutamic acid, and proline mutations were more potent than threonine in inducing cell transformation. The growth of cells transformed by CSF-1R [S301] and [T301] was further stimulated by human recombinant CSF-1, but cells expressing CSF-1R [E301] responded poorly to the growth factor. The transforming efficiency of mutant receptors was also enhanced by the presence of a phenylalanine for tyrosine mutation at codon 969 near the receptor carboxylterminus. Like the v-fms oncogene product, receptors containing S301 or E301 mutations were partially inhibited in their intracellular transport to the plasma membrane, whereas non-transforming variants were transported normally. However, CSF-1R [T301] was processed as efficiently as the wild-type glycoprotein, indicating that the properties of altered transport and cell transformation could be at least partially dissociated. Expression of CSF-1 receptors bearing activating mutations led to increased phosphorylation of cellular substrates on tyrosine, suggesting that cell transformation resulted from constitutive receptor kinase activity. We conclude that only particular mutations at codon 301 mimic an effect of ligand on CSF-1R so as to constitutively activate its growth promoting activity.