We have identified a second baby with the fetal methemoglobin F-M-Fort Ripley. It was observed in a Caucasian infant from Canada; at least eleven additional members of that family were known to have had a neonatal cyanosis similar to that seen in the propositus and in a previously described baby (2). Sequencing of amplified DNA that included (part of) the Gγgene greatly facilitated the characterization. The Gγx chain was readily isolated by reversed phase high performance liquid chromatography; its quantity was∼12.5% of totalγ. Interestingly, the baby also carried the AγT mutation on one chromosome, either in cis or in trans to the Gγx mutation. Hb F-M-Fort Ripley could be isolated in reasonably pure form by DEAE-cellulose chromatography. The isolated Hb Fx was unstable, had spectral changes characteristic for the M-hemoglobins, while its methemqglobin derivative reacted rapidly with cyanide. Oxygen affinity data could not be obtained. It is suggested that the formation of a rather large amount (∼25%) of mixed hybrids (α2GγX.γ) with low oxygen affinity is the main cause for the occurrence of the neonatal cyanosis.