The incidence of spontaneous mammary carcinomas was determined in the C3H mouse inbred colony reared in Berlin-Buch over the past three years. Only female animals reaching a lifespan of no less than 170 days, corresponding to the longevity of the shortest lived animal in which a mammary carcinoma developed, were included in the study. A 50% incidence of mammary carcinomas was found with female animals above one year of age. The median age for the incidence of tumours was 446 days. Related to tumour-free females which at least have reached the median tumours age, the proportion of females developing mammary carcinomas was 86.8%. With increasing litters the tumour latency decreased from 491 for monoparous females to 383 days for mothers with 7 to 8 litters. The growth rate of 60 tumours under observation was found to vary grossly. With the minimum volume doubling time being the same in all cases, the growth rate of the tumours varied according to longer or shorter phases of stagnation or delay. Tumours having approximately equal growth rates throughout the period of investigation were classified as a "growth type". Altogether, seven growth types for spontaneous mammary carcinomas were established.