Dimethylbenz(alpha)anthracene (DMBA)-induced and transplanted rat mammary tumours (2 lines) were examined for oestrogen receptor activity, and for sensitivity to hormones in vivo (by ovariectomy) and in vitro (by tissue culture). In vivo, the growth of all tumours induced by the administration of DMBA in random-bred Sprague-Dawley rats was found to be dependent on the ovary, whilst in all transplanted tumours (12 TG-3 and six TG-5 lines), maintained in an inbred strain of Sprague-dawley rats, growth was found to be independent of the ovary. In vitro, the capacity for DNA synthesis in DMBA-induced tumours was better maintained after 24 h when insulin (10 microgram/ml) and corticosterone (5 microgram/ml) or insulin, corticosterone and prolactin (each 5 microgram/ml) were present in the medium (five out of 12 and eight out of 11 tumours respectively); no effect of hormones in the media was detected after 48 h. In the transplanted tumours, no effect of hormones on DNA synthesis was detected after either 24 or 48 h of culture. Synthesis of lecithin was not detectably influenced by the presence of hormones in either DMBA-induced or transplanted tumours. Oestrogen receptor concentrations were, on average, significantly higher in the DMBA-induced tumours than in either line of transplanted tumour. For 22 DMBA-induced tumours and 15 transplanted tumours, the effect of hormones in vitro ('response') was directly correlated with receptor concentration at time 0 (Spearman's rho = +0.59) and inversely correlated with the rate of DNA synthesis ('basal') at time 0 (Spearman's rho = -0.62). No single parameter or pair of parameters permitted accurate distinction between the tumour types.